Ceria-based mixed-metal oxide structure, including method of making and use

ABSTRACT

A homogeneous ceria-based mixed-metal oxide, useful as a catalyst support, a co-catalyst and/or a getter has a relatively large surface area per weight, typically exceeding 150 m 2 /g, a structure of nanocrystallites having diameters of less than 4 nm, and including pores larger than the nanocrystallites and having diameters in the range of 4 to about 9 nm. The ratio of pore volumes, V P , to skeletal structure volumes, V S , is typically less than about 2.5, and the surface area per unit volume of the oxide material is greater than 320 m 2 /cm 3 , for low internal mass transfer resistance and large effective surface area for reaction activity. The mixed metal oxide is ceria-based, includes Zr and or Hf, and is made by a novel co-precipitation process. A highly dispersed catalyst metal, typically a noble metal such as Pt, may be loaded on to the mixed metal oxide support from a catalyst metal-containing solution following a selected acid surface treatment of the oxide support. Appropriate ratioing of the Ce and other metal constituents of the oxide support contribute to it retaining in a cubic phase and enhancing catalytic performance. Rhenium is preferably further loaded on to the mixed-metal oxide support and passivated, to increase the activity of the catalyst. The metal-loaded mixed-metal oxide catalyst is applied particularly in water gas shift reactions as associated with fuel processing systems, as for fuel cells.

This application is a division of U.S. patent application Ser. No.10/402,808 filed Mar. 28, 2003, which in turn is a continuation-in-partof U.S. patent application Ser. No. 10/109,161 filed Mar. 28, 2002.

TECHNICAL FIELD

This invention relates to mixed metal oxides, and more particularly toceria-based mixed-metal oxide structures, for use as catalyst supports,as co-catalysts, as getters, and the like. The invention relates furtherto methods of preparing such ceria-based mixed-metal oxide structures,and further still to metal loading of such structures. The inventionrelates still further to the application of such mixed-metal oxidestructures as catalyst supports, co-catalysts, and/or getters in, forinstance, fuel processing systems.

BACKGROUND ART

Various metal oxides have found use in chemically reactive systems ascatalysts, supports for catalysts, gettering agents and the like. Asused herein, a gettering agent, or getter, is a substance that sorbs orchemically binds with a deleterious or unwanted impurity, such assulfur. In those usages, their chemical characteristics and morphologiesmay be important, as well as their ease and economy of manufacture. Onearea of usage that is of particular interest is in fuel processingsystems. Fuel processing systems catalytically convert hydrocarbons intohydrogen-rich fuel streams by reaction with water and oxygen. Theconversion of carbon monoxide and water into carbon dioxide and hydrogenthrough the water gas shift (WGS) reaction is an essential step in thesesystems. Preferential oxidation (PROX) of the WGS product using suchcatalysts may also be part of the process, as in providing hydrogen fuelfor a fuel cell. Industrially, copper-zinc oxide catalysts, oftencontaining alumina and other products, are effective low temperatureshift catalysts. These catalysts are less desirable for use in fuelprocessing systems because they require careful reductive activation andcan be irreversibly damaged by air after activation.

Recent studies of automotive exhaust gas “three-way” catalysts (TWC)have described the effectiveness of a component of such catalysts, thatbeing noble metal on cerium oxide, or “ceria” (CeO₂), for the water gasshift reaction because of its particular oxygen storing capacity (OSC).Indeed, the ceria may even act as a “co-catalyst” with the noble metalloading in that it, under reducing conditions, acts in concert with thenoble metal, providing oxygen from the CeO₂ lattice to the noble metalsurface to oxidize carbon monoxide and/or hydrocarbons adsorbed andactivated on the surface. In many cases the ceria component of thesecatalysts is not pure ceria, but cerium oxide mixed with zirconium oxideand optionally, other oxides such as rare earth oxides. It has beendetermined that the reduction/oxidation (redox) behavior of the ceriumoxide is enhanced by the presence of ZrO₂ and/or selected dopants.Robustness at high temperatures is an essential property of TWC's, andthus, such catalysts do not typically have either sustainable highsurface areas, i. e., greater than 100 m²/g, or high metal dispersion(very small metal crystallites), even though such features are generallyrecognized as desirable in other, lower temperature, catalyticapplications.

For mixed-metal oxides that are to be used as such catalyst supports andwhich comprise cerium oxide and zirconium and/or hafnium oxide, it isgenerally desirable that they possess a cubic structure. The cubicstructure is generally associated with greater oxygen mobility, andtherefore greater catalytic activity. Moreover, the zirconium and/orhafnium provide thermal stability, and thus contribute to the thermalstability and life of a catalyst. Yashima et al., in an article entitled“Diffusionless Tetragonal-Cubic Transformation Temperature in ZirconiaSolid Solution” in Journal of American Ceramic Society, 76 [11], 1993,pages 2865-2868, have recently shown that cubic ceria undergoes a phasetransition to tetragonal when doping levels of zirconia are at or above20 atomic percent. They suggest that above 20 percent zirconia, theoxygen anion lattice distorts into a tetragonal phase, while the ceriumand zirconium cations remain in a cubic lattice structure, creating anon-cubic, metastable, pseudo-tetragonal phase lattice. Traditionally,powder X-ray diffraction (PXRD) is used to identify the structure andsymmetry of such phases. However, in the case of ceria-zirconia oxideswith very small crystallite sizes (i.e., less than 3 nm), the PXRDsignal exhibits broadened peaks. Additionally, the signal produced bythe oxygen atoms, which is a function of atomic weight, is drowned outby the intense signal produced by the cerium and zirconium cations. Thusany tetragonal distortion, caused by the oxygen atoms shifting in thelattice, goes unnoticed in a PXRD pattern and the resulting patternappears cubic. In such cases, Raman spectroscopy and X-ray absorptionfine structure (EXAFS) can be employed to observe such phasetransitions. Yashima et al. have published Raman spectroscopy and EXAFSstudies in support of the position taken above. Vlaic et al., in anarticle entitled “Relationship between the Zirconia-Promoted Reductionin the Rh-Loaded Ce_(0.5)Zr_(0.5)O₂ Mixed Oxide and the Zr—O LocalStructure” in Journal of Catalysis, 168, (1997) pages 386-392, haveshown similar results for a phase transition at 50% zirconia, asdetermined by Raman spectroscopy and EXAFS.

Ceria-zirconia mixed oxide materials having relatively large surfaceareas per unit weight may be particularly well suited in variouscatalytic and/or gettering (i.e., sulfur sorbing) applications, as mightbe typified by, but not limited to, the WGS reaction. Indeed, suchceria-based mixed metal oxides may be used first in a WGS system as agetter to adsorb sulfur-containing compounds from the gas stream toprotect more sensitive/valuable components downstream that use suchoxides as catalysts in the WGS reaction. In that general regard, it isdeemed desirable that the mixed oxide material be comprised of smallcrystallites agglomerated to form porous particles having relativelylarge surface areas per unit weight as a result of significant porediameters and pore volumes. Large pore diameters facilitate masstransfer during catalytic reactions or gettering applications, byminimizing mass transfer resistance. On the other hand, excessive porevolumes may act to minimize the amount of effective surface area in agiven reactor volume, for a given final form of catalyst or getter,thereby limiting the catalytic or gettering action in a given reactorvolume. Thus, the ratio of pore volume to the structural mass, as wellas crystallite size and pore diameters, can be optimized within a range.In this regard then, the particular morphology of the ceria-basedmixed-metal oxide material becomes important for efficient operation ofthe material as a catalyst or getter in particular reactions and/orunder particular operating conditions and geometries.

A variety of synthesis techniques have been used to provideceria-zirconia mixed oxide materials. These techniques includeconventional co-precipitation, homogeneous coprecipitation, the citrateprocess, and a variety of sol-gel techniques. However, as far as can bedetermined, the surface areas of the mixed metal oxides resulting fromthese techniques are typically less than about 130 m²/g. Liquid phasesynthesis at relatively low temperatures is preferred, as it allows forthe formation of metastable phases and offers the ability to controlsuch properties as surface area, particle size, and pore structure.Typical solution routes have involved two steps, hydration andcondensation. It has been generally accepted that the gel matrix formedupon hydration is amorphous and only forms a crystalline structure whenthe framework undergoes condensation. While hydration occurs at themoment the gelatinous phase is formed from solution, condensation hasusually been expected to occur during the aging (maturing), dryingand/or calcinations steps. For many mixed metal oxide systems, thedetailed conditions under which these steps (such as aging) occur are,and have been, critical parameters in determining the properties of thefinal product. Thus, a time consuming step such as aging has beenessential.

Surface areas as great as 235 m²/g for such materials have been reportedby D Terribile, et al, in an article entitled “The preparation of highsurface area CeO2—ZrO2 mixed oxides by a surfactant-assisted approach”appearing in Catalysis Today 43 (1998) at pages 79-88, however, theprocess for their production is complex, sensitive, and time-consuming.The process for making these oxides requires the use of a surfactant anda lengthy aging, or maturing, interval of about 90 hours at 90° C.Moreover, the initial precipitate must be washed repeatedly with waterand acetone to remove the free surfactant (cetyltrimethylammoniumbromide) before the material can be calcined, thereby contributing todelays and possible other concerns. Still further, the mean particlesizes of these oxides appear to be at least 4-6 nm or more. The porevolume is stated to be about 0.66 cm³/g. This relatively large porevolume per gram is not consistent with that required for a ceria-basedmixed metal oxide which, while thermally robust, should tend to maximizeboth the available surface area in a given reactor volume and the masstransfer characteristics of the overall structure as well as theappropriate reactivity of that surface area, as is desired in theapplications under consideration. Assuming the density, D, of thismaterial is about 6.64 g/cm³, the skeleton has a volume, V_(S), of 1/D,or about 0.15 cm³/g, such that the total volume, V_(T), of one gram ofthis material is the sum of the pore volume, P_(V), (0.66 cm³/gm) andthe skeletal volume, V_(S), which equals about 0.81 cm³/gm. Hence, 235m²/gm÷0.81 cm³/gm equals about 290 m²/cm³. Because of the relativelylarge pore volume, the surface area per unit volume of a material ofsuch density has a reduced value that may not be viewed as optimal.

For use of a mixed-metal oxide in a catalyst application, it is requiredto be loaded with a metal, such as a noble metal, providing goodcatalytic activity to the media being processed. While noble metals suchas platinum have provided good catalytic activity, it is alwaysdesirable to improve the activity, cost, and/or durability of suchcatalyst metal loadings.

It is desirable to provide ceria-based mixed-metal oxide materialshaving the aforementioned positive properties and avoid the limitations,for use in catalytic reactions/gettering applications generally, andfuel processing catalytic reactions/gettering applications morespecifically. Even more particularly, it is desirable to provide suchceria-based mixed-metal oxide materials for use in, for example, watergas shift reactions employed in fuel processing systems for theproduction of hydrogen-rich feed stock.

Accordingly, it is an object of the invention to provide ceria-basedmixed metal oxides having the aforementioned desirable properties ofrelative stability, high surface areas, relatively small crystallites,and pore volumes sized to optimally balance the reduction of masstransfer resistance with the provision of sufficiently effective surfaceareas in a given reactor volume, particularly for use as a catalystsupport or co-catalyst, though not limited thereto.

It is another object of the invention to provide such ceria-basedmixed-metal oxides having enhanced redox capability, and moreoverpossessing good thermal stability.

It is an even further object of the invention to provide a catalystincluding the ceria-based mixed metal-oxide as a support, in accordancewith the forgoing objects. Further to this object, it is desired toprovide the support with a catalyst metal loading that exhibits enhancedactivity, cost, and/or durability.

It is a still further object of the invention to provide an efficientand economical process for making such ceria-based mixed-metal oxidecatalyst supports, catalysts, and/or getters in accordance with theforegoing objects.

It is yet a further object of the invention to utilize a catalystemploying a ceria-based mixed metal oxide as a support, co-catalyst, orgetter, made in accordance with the foregoing objects, in a fuelprocessing system in, for example, a water gas shift reaction.

DISCLOSURE OF INVENTION

The present invention relates to a ceria-based mixed-metal oxidematerial, and more particularly to such material having a relativelyhigh surface area per unit of weight, relatively small crystallitediameters, pore diameters of the crystallites in the aggregate thatnormally exceed the crystallite diameters, and having an aggregatedcrystallite morphology that is thermally robust, and that optimizes theavailable surface area per unit volume, mass transfer characteristics,and the reactivity of that surface area. The invention also relates tothe selection of metal constituents in the metal oxide mix with theceria base, for providing the aforementioned characteristics, and maypreferably include one or more of the relatively redox tolerant ionsZr⁺⁴, Hf⁺⁴ and Ti⁺⁴, rare-earth ions such as typical lanthanide ionsLa⁺³ and Yb⁺³ and non rare-earth metal ions such as Mo⁶⁺ and Ta⁵⁺.

The invention further relates to the process(es) for making suchceria-based oxides, to the use of such ceria-based mixed-metal oxides ascatalyst supports, co-catalysts, getters and the like, and to thecatalyst metal supported thereby and the process for its manufacture.The invention also relates to the use of such ceria-based mixed metaloxide supports and catalysts particularly in water gas shift (WGS)and/or preferential oxidation (PROX) reactions in fuel processingsystems, as for example fuel cells.

According to the invention, there is provided a material of homogeneouscerium-based binary, ternary or quaternary mixed-oxides that arenano-crystalline, have an average crystallite size less than 4 nm aftercalcinations at 500° C. or less, and which after calcination in air for1-6 hours, and preferably 2-4 hours, at temperatures in the range ofabout 250°-600° C., and preferably 350°-500° C., have high (large)surface areas greater than 150 m²/g, a skeletal density of about 6.5g/cm³, pore volumes of moderate size such that the surface area per unitvolume of the porous material is greater than 320 m2/cm3, and preferablygreater than 420 m²/cm³, and an average pore diameter of the aggregated(agglomerated) nanocrystallites normally greater than thenanocrystallites, typically being greater than 4 nm but less than about9 nm in keeping with pore volumes of moderate size. As used herein, theterm “homogeneous” refers to the elemental composition of the individualnanocrystallites that reflects the overall elemental composition.

The ceria-based mixed-metal oxide nanocrystalline material described inthis invention exhibits a fractal morphology, thus leading to aminimization in internal mass transfer resistance. Within the context ofa nanoscale material, a pattern formed within an aggregate that isregular and repeating at increasing magnitudes of scale can beconsidered as fractal. Having a fractal structure eliminates the need todesign a catalyst material with very large pore structures. If fractal,the material possesses larger, 100 to 200 nm and micron-sized pores inthe aggregate (inter-particle void space, as opposed to intra-particlevoid space that defines the 4-9 nm pore diameters described above) whichprovide enough open space for gas molecules to diffuse in and react. Byreducing the internal pore diameter (i. e., the inter-particle voidspace) to a smaller size, the internal surface area may be increased,leading to a larger number of active sites per volume and a thus ahigher catalytic activity.

This combination of surface area and average pore diameter translatesinto relatively low internal mass transfer resistance. However, if thatvalue becomes too small because of excessive pore size and/or volume,the effective number of sites per crystallite aggregate necessarilydecreases and the amount of effective surface area per unit reactorvolume also decreases. As described earlier, for a porous material ofgiven density, D, the skeletal volume, V_(S), is 1/D, such that thetotal volume of a gram of material, V_(T), is the sum of the porevolume, V_(P), +skeletal volume, V_(S). From this, the surfacearea/gram/V_(T) yields the surface area per unit volume of material, andit is this value which the invention seeks to maximize. Accordingly, ithas been determined that the surface area per unit volume of materialshould be greater than 320 m²/cm³, and preferably greater than 420m²/cm³. In this respect, because the pore diameter and pore volume arerelated, it has been determined that the pore diameter should bemoderate and in the range of more than 4 nm but less than 9 nm. Viewedyet another way, it has been determined that the ratio of pore volume,V_(P), to the particle, or skeletal volume, V_(S), should not exceedabout 2.5.

In addition to the cerium oxide, the other oxides in the mix are derivedfrom one or more constituents from the group which includes Zr(zirconium), Hf (hafnium), Nb (niobium), Ta (tantalum), La (lanthanum),Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu (europium), Gd(gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium),Tm (thullium), Yb (ytterbium), Lu (lutetium), Mo (molybdenum), W(tungsten), Re (rhenium), Rh (rhodium), Sb (antimony), Bi (bismuth), Ti(titanium), V (vanadium), Mn (manganese), Co (cobalt), Cu (copper), Ga(gallium), Ca (calcium), Sr (strontium), and Ba (barium).

The composition of the bulk mixed metal oxide is: cerium, between 40%and 85%; zirconium or hafnium, or mixtures thereof, between 15% and 60%;one or members of the group: Ti, Re, Nb, Ta, Mo, W, Rh, Sb, Bi, V, Mn,Co, Cu, Ga, Ca, Sr, and Ba, between 0% and 10%; and one or more membersof the group: Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu,between 0% and 10%; and where all percentages are on a metals-onlyatomic basis. In accordance with one aspect of the invention, the ceriumis between about 40% and 70%, and the zirconium or hafnium, or mixturethereof, is between about 25% and 60% and most preferably is greaterthan about 45%. Under another aspect of the invention, the cerium may be60% or more. Moreover, the mixed metal oxide exhibits, via Ramanspectroscopy, a cubic structure to the effective exclusion of thetetragonal phase, over all, or most, of the compositional range ofinterest for the several embodiments. For purposes of the discussionherein, if a constituent in the mixed metal oxide is present in anamount less than or equal to about 10% of the total, it may be referredto as a “dopant”. It will be understood by those skilled in the art thatnot all of the listed dopants are equally effective or even desirablefor all processes in which these ceria-based oxides may be used. Forinstance, some dopants such as Ga and Bi may not be desirable inPt/ceria-zirconia catalysts if for use in WGS reactions.

The inventive process for making the ceria-based mixed metal oxidematerials having the constituents, properties and morphology of theinvention avoids the need for using surfactants and lengthy aging steps,and includes the steps of 1) dissolving salts of the cerium and at leastone other constituent in water to form a dilute metal salt solution; 2)adding urea, either as a solid or aqueous solution; 3) heating thesolution of metal salt and urea to near boiling (which may includeboiling) to coprecipitate homogeneously a mixed-oxide of the cerium andthe one or more other constituent(s) as a gelatinous coprecipitate; 4)optionally maturing, if and when beneficial, the gelatinouscoprecipitate in accordance with a thermal schedule; 5) replacing waterin the solution with a water miscible, low surface-tension solvent, suchas dried 2-propanol; 6) drying the coprecipitate and solvent to removesubstantially all of the solvent; and 7) calcining the driedcoprecipitate at an effective temperature, typically moderate, for aninterval sufficient to remove adsorbed species and strengthen thestructure against premature aging. In the dilute metal salt solution,the metal concentration is less than 0.16 mol/L, is preferably less thanabout 0.02 mol/L, and is most preferably less than about 0.016 mol/L,and the urea concentration is relatively high, being greater than 0.25mol/L and preferably about 0.5 to 2.0 mol/L. The maturing of thecoprecipitate, if even required at all, is accomplished in less than 72hours, and preferably less than about 24 hours, for example in the rangeof 3 to 8 hours. Indeed, it has recently been discovered thatcrystallization may, and often does, occur when the gelatinouscoprecipitate is formed, thus further reducing or eliminating the needfor maturing the coprecipitate, particularly depending upon the end useapplication of the material. The calcining of the dried coprecipitateoccurs for 1-6 hours, and preferably 2-4 hours, at a heating rate in therange of about 2°-10° C./min with a final calcining temperature in therange of 250°-600° C., and preferably in the range of 350°-500° C.

It has been further discovered that the surface area of the ceria-basedmixed-metal oxide material is, to some extent, a function of the gasatmosphere condition under which the material is calcined. A flowing gasis preferable to a static gas condition, air provides an economic sourceof a flowing gas that provides good results, and calcining the oxidematerial under flowing CO₂, or more preferably under CO₂ mixed with adilute O₂ mixture, such as a mixture ranging from 20% CO₂; 40% O₂; and40% Ar to 80% CO₂; 10% O₂; and 10% Ar, or even more preferably under CO₂followed by such a mixture of dilute O₂, but without CO₂, appears toyield particularly large surface areas, in excess of 250 m²/g.

The ceria-based mixed-metal oxide material of, and made in accordancewith, the invention finds particular utility as a catalyst support in acatalytic fuel processing system. A highly dispersed catalyst metal isloaded on the described mixed-metal oxide support to a concentration inthe range of 0.1 to 6.0 wt %. The catalyst metal is chosen to havecrystallites that are predominantly less than 2.5 nm in size, andpreferably less than 2.0 nm. The catalyst metal may typically be a noblemetal, with platinum being preferred.

Although the catalytic activity afforded by Pt is relatively high andeffective for many processes, it has been discovered that the additionof rhenium (Re) with the loading of the noble metal (e.g., Pt) on themixed-metal oxide support yields a water gas shift and/or PROX catalystof particularly high activity. The turnover rate (TOR—the rate persecond at which Moles of CO are converted per Mole of Pt) issignificantly greater for such catalysts that include Re relative tothose that have Pt without Re. The Re is loaded, to a concentration inthe range of 0.5 to 6.0 wt %, on the mixed metal oxide supportpreviously loaded with the catalyst noble metal.

The process for loading the catalyst comprises the steps of 1) surfacetreating the support in a solution containing an acid from the groupconsisting of amino acids, hydroxy dicarboxylic acids, hydroxypolycarboxylic acids, and keto polycarboxylic acids; and 2) loading thecatalyst metal by submerging the surface-treated support in a solutioncontaining the catalyst metal. The acid used for surface treating thesupport is preferably malic acid or citric acid. The solution containingthe catalyst metal may be a solution of tetraamineplatinum nitratehaving about 1 weight percent platinum, 1 weight percent ammoniahydroxide and 15 weight percent 2-propanol, and the surface-treatedsupport is submerged therein for about 2 hours at room temperature,following which it is filtered and dried. The catalyst-loaded support isthen calcined for up to 4 hours at a heating rate of about 2° C./min toa calcining temperature in the range of 250°-600° C., and morepreferably in the range of 350°-500° C. The resulting catalyst is thenused, in accordance with another aspect of the invention, in a water gasshift reactor and/or a preferential oxidizer in a fuel processingsystem.

In accordance with an aspect of the catalyst loading process of theinvention, it has been found to be particularly desirable to carefullytailor the surface treatment step to the acid being used and theparticular composition of the ceria-based mixed-metal oxide support, inorder to accomplish the desired surface treatment without excessivedegradation of the oxide support material. In this regard, thenanocrystalline mixed metal oxide is first titrated with the acid to beused in order to establish a titration curve having an equivalence pointat which the pH stabilizes despite the continued addition of the titrantacid. The titration curve, or a family of such curves for differingacids and/or support materials, is then used to optimize the surfacetreatment process.

In accordance with a further aspect of the catalyst loading process ofthe invention, there is provided a preferred process for loading the Reon to the noble metal-loaded mixed-metal oxide. The source of the Re isnot particularly critical, and may include ammonium perrhenate(NH₄ReO₄), perrhenic acid (HReO₄), rhenium carbonyl (Re₂(CO)₁₀), or thelike, with either of the first two mentioned examples having a costadvantage. The noble metal-loaded nanocrystalline mixed metal oxide ofthe invention is immersed in an appropriate solvent; water or a watercontaining mixture, is an excellent solvent for the ammonium perrhenate(NH₄ReO₄) or perrhenic acid (HReO₄), while an organic solvent liketetrahydrofuran is an excellent solvent for rhenium carbonyl (Re₂(CO)₁₀)in this application. After an optional degassing or inert gas purgingstep, the noble metal-loaded, preferably Pt-loaded, nanocrystallinemixed metal oxide is contacted with a hydrogen containing gas to reduceand/or remove chemisorbed oxygen from the surface of the noble metal.Separately, the Re source material in the amount sufficient to add thedesired amount of Re to the noble metal-loaded nanocrystallinemixed-metal oxide is combined with the solvent to form a solution. Thissolution then replaces, or is added to, the solvent contacting the solidsuch that the noble metal-loaded mixed-metal oxide is contacted with theRe source-containing solution. Contact with the hydrogen-containing gasis continued to reduce the perrhenate ion, which in turn results in aclose association of the Re with the Pt. If rhenium carbonyl is used,the interaction with the noble metal under hydrogen is believed toresult in the decomposition of the rhenium carbonyl, thus depositing Reon the noble metal. As one skilled in the art will recognize, therhenium carbonyl can be replaced with another reasonably labile rheniumcompound/complex or an organometallic rhenium compound free of known orsuspected elements deleterious to the catalyst. The mixture is thenstirred under the H₂ flow for a period of time, followed by a switch toan inert gas. After the hydrogen gas is substantially removed, oxygen orair may be gradually introduced to the inert gas with care being takenthat the temperature is maintained below 50° C., preferably below about30° C. It is also preferable to remove all, or nearly all, of anyflammable solvent before the oxygen is introduced. This passivation stepis important to prevent pyrophoric ignition upon contact with air, andmay be accomplished using alternative equivalent passivation techniques.

While varying the amount of catalytic material or loading of an activemetal to suit a particular feed stream is not unknown in the art ofcatalysis, varying the composition of a catalyst within a catalystfamily for a particular feed stream to optimize its performance isunusual. It has been discovered that for a given reformate stream thatis a mixture of CO, CO₂, H₂, H₂O and other gases, where the H₂O/CO ratiois less than about 6, that a series of noble metal-loadednanocrystalline mixed metal oxides have similar CO conversion to CO₂turnover rates over a range of cerium to zirconium or hafnium ratios,but surprisingly for H₂O to CO ratios between 6-30, keeping all otherreactant and product concentrations fixed, differences in WGS activityemerge with differences in cerium-to-zirconium or hafnium ratios. Thus,it has been discovered that the WGS catalyst composition may be“tailored” to maximize its activity for a particular range of feed gascomposition, and likewise, that the feed gas composition may be tailoredthrough the addition of water and/or the removal of CO₂ and/or H₂ tooperate in the regime of maximum catalyst activity. Furthermore, thisaspect of the invention also covers the use of a catalyst bed whereeither the cerium-to-zirconium or hafnium ratio changes or the Pt-to-Reratio changes, or both, along the catalyst bed to optimize performance.

According to this aspect of the invention, there is provided a method ofoptimizing water gas shift activity for a water gas shift reaction on areformate, or reformate range of interest, in the presence of a shiftcatalyst. The method comprises the steps of: determining certaincompositional characteristics of one or more reformates for a range ofreformate compositions comprising a reformate range of interest;determining the respective activity rates for a range of shift catalystcompositions relative to the reformate range of interest; and selectingfor the water gas shift reaction, from the range of shift catalystcompositions, a shift catalyst composition having a favorable activityrate for the reformate range of interest or alternatively, from thereformate range of interest, a reformate composition providing afavorable activity rate to a predetermined shift catalyst composition.Within this method, the range of shift catalyst compositions comprises arange of atomic % ratios of Ce to one or both of Zr and Hf for aceria-based mixed-metal oxide catalyst support. This range of shiftcatalyst compositions may also comprise a range of weight % ratios of Ptto Re, or a range of shift catalysts where both the oxide compositionand the metal loading composition is optimized to the feed compositionof the expected gas composition at a given point in a conversionreactor. Further, the range of reformate compositions comprises at leastH₂O to CO partial pressure ratios.

While not wishing to be bound to the following theory, it is believedthat the relationship described above is due to changes in the catalystkinetic rate expression with composition, and that these changes reflectamong other factors the degree of reduction of the oxide, and therelative surface population of CO, H₂O and CO₂ species, or speciesderived therefrom, on the surface of the nanocrystalline oxide as thecomposition of the reformate changes. In the nanocrystalline material ofthe invention where the nanocrystals are only 4 to 6 unit cells indiameter, the change surface species can change the subsurfacestructure. This in turn impacts the oxide ion conductivity andelectronic conductivity. Changes in the oxide composition, such aschanges in the Ce to Zr ratio or the addition of dopants magnify ormitigate these changes through their influence of the relative surfacepopulations and on the equilibrium degree of reduction of the oxideimposed by given reformate composition present over the nanocrystallineoxide at a given temperature and noble metal composition and loading.

The foregoing features and advantages of the present invention willbecome more apparent in light of the following detailed description ofexemplary embodiments thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a powder X-ray diffraction pattern of a ceria-zirconia swollengel, as formed in solution in accordance with the invention;

FIG. 2 is a pair of comparative electron micrographs showing the minimalimpact of aging time on ceria-zirconia nanocrystalline material inaccordance with the invention;

FIG. 3 is a powder X-ray diffraction pattern for a Ce_(0.5)Zr_(0.5)O₂nanocrystalline support material of the invention, fit to both cubic anttetragonal symmetry;

FIG. 4 is a Raman spectra of a Ce_(0.65)Zr_(0.35)O₂ nanocrystallinesupport material in accordance with the invention;

FIG. 5 depicts Raman spectra for each of several levels of zirconia upto 50% in the ceria oxide nanocrystalline support material;

FIG. 6 is a Raman spectra of a Ce_(0.41)Zr_(0.59)O₂ support materialindicating the absence of the tetragonal phase;

FIG. 7 depicts Raman spectra for ceria-zirconia nanocrystalline supportmaterial of the invention prepared at various calcination temperatures;

FIG. 8 is an electron micrograph displaying the fractal morphology ofnanocrystalline ceria-zirconia material in accordance with theinvention;

FIG. 9 is a titration curve for Sample FF3B of Example 28;

FIG. 10 is a titration curve for Sample UR201C of Example 29;

FIG. 11 is a titration curve for Sample UR149 of Example 31;

FIG. 12 is a titration curve for Sample 201E of Example 33;

FIG. 13 is a titration curve for Sample UR201A of Example 34;

FIG. 14 is a titration curve for Sample UR201J of Example 35; and

FIG. 15 is a titration curve for Sample UR202 of Example 36.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention relates to a ceria-based mixed-metal oxide material,useful as a catalyst support, a co-catalyst and/or a getter, and to thecatalyst metal supported thereby in the instance of catalyst usage. Theinvention also relates to the processes associated with making suchceria-based mixed metal-oxide materials, as supports, catalysts, and/orgetters (i.e., sulfur sorber). The invention further relates to use ofsuch ceria-based mixed metal oxides as catalysts, or catalyst supports,in fuel processing systems. As used herein, a supported catalyst, orsimply catalyst, comprises the combination of a catalyst support and acatalyst metal dispersed thereon. The catalyst metal may be referred toas being loaded on to the catalyst support, and may, in instancesherein, be referred to simply as “the catalyst”, depending on thecontext of usage. Because the ceria-based mixed metal oxide material andprocess of the invention finds particular utility as a catalyst support,though is not limited to such use, the following discussion of thatoxide material and the process by which it is made is in the context ofsuch a support. Thus, reference to “the support” is synonymous with theoxide material of the invention and will typically be used forsimplicity.

It is desirable to efficiently maximize the effective surface area of acatalyst support, particularly for use in water gas shift (WGS)reactions and/or preferential oxidation (PROX) reactions to processhydrocarbon feedstocks into hydrogen-rich fuels for fuel cells, in orderto make the resulting reaction as efficient as possible. Consequently,the proper combination of relatively high surface area per unit skeletaldensity coupled with relatively, though not excessively, large poresthat minimize internal mass transfer resistance without creatingexcessive pore volume, results in a highly effective catalyst thatincreases catalyst efficiency by maximizing the amount of effectivesurface area within a given reactor volume. By increasing the efficiencyof a catalyst in such a reaction, it is possible then to either increasethe reaction flow for a given reactor volume or to decrease the reactorvolume for a given reaction flow, or a combination of the two. The useof such fuel processing systems in mobile applications placesconsiderable emphasis on reducing size/volume, as will be understood.The improved catalyst support/catalyst/getter of the inventioncontribute to this objective.

The process(es) and product(s) of the invention involve the formation ofhigh surface area ceria-based mixed-metal oxides as catalyst supportsand catalysts of the type particularly suited for use in WGS reactionsand PROX reactions, as for the fuel processing system associated withproviding a hydrogen-rich fuel supply to a fuel cell. Moreover, thesupports and catalysts are formed by processes that are efficient andeffective. Consideration will first be given to the formation of thehigh surface area ceria-based mixed-metal oxide material of the catalystsupport, and then to the formation/loading of the catalyst metal on thatsupport.

The support is a homogeneous structure of cerium oxide and at least oneother metal oxide constituent that are all nano-crystalline, that is,less than about (<) 4 nm. For binary mixed-metal oxides, the otherconstituents preferably are selected from the group consisting ofzirconium and hafnium. Further still, some advantage may be derived fromhomogeneous ternary and/or quaternary cerium-zirconium or cerium-hafniumbased mixed oxides that provide the same nano-crystalline structure. Inproviding the homogeneous ternary and/or quaternary mixed oxides ofcerium and either zirconium or hafnium, the additional constituents, ordopants, are selected from the group of metals consisting of rare earthmetals La (lanthanum), Pr (praseodymium), Nd (neodymium), Sm (samarium),Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), Tm (thullium), Yb (ytterbium), Lu (lutetium),and non-rare earth metals Cu (copper), Mo (molybdenum), W (tungsten), Nb(niobium), Ta (tantalum), Re (rhenium), Rh (rhodium), Sb (antimony), Bi(bismuth), V (vanadium), Mn (manganese), Co (cobalt), Ga (gallium), Ca(calcium), Sr (strontium), Ba (barium), and Ti (titanium). Thecomposition of the bulk mixed metal oxide is: cerium, between 40% and85%; zirconium or hafnium, or mixtures thereof, between 15% and 60%; oneor members of the group: Ti, Re, Nb, Ta, Mo, W, Rh, Sb, Bi, V, Mn, Co,Cu, Ga, Ca, Sr, and Ba, between 0% and 10%; and one or more members ofthe group: Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu,between 0% and 10%; and where all percentages are on a metals-onlyatomic basis. In accordance with one aspect of the invention, the ceriumis between about 40% and 70%, and the zirconium or hafnium, or mixturethereof, is between about 25% and 60% and most preferably is greaterthan about 45%. Under another aspect of the invention, the cerium may be60% or more. Moreover, the mixed metal oxide preferably exhibits a cubicstructure over the compositional range of interest for the severalembodiments.

It will be understood and appreciated that by using the process of theinvention, these cerium-based mixed-metal oxides form support structuresthat have a nano-crystalline structure that, on average, is less thanabout 4 nm, as determined by powder x-ray diffraction line broadening; ahigh B. E. T. surface area that exceeds 150 m²/g after calcination atabout 400° C. for about 4 hours, and typically is about 180 m²/g ormore; and relatively large pores as determined by nitrogen adsorption,the average pore size as determined by the maximum in the pore sizedistribution curve for the material being greater than 4 nm, andtypically 5 nm or greater, to about 9 nm, and thus normally larger thanthe crystallite size. These characteristics tend to maximize andoptimize the surface for interaction with the gas phase, by combiningrelatively high surface area per unit skeletal density with relatively,though not excessively, large pores that minimize internal mass transferresistance without creating excessive pore volume, to result in a highlyeffective catalyst that increases catalyst efficiency by maximizing theamount of effective surface area within a given reactor volume. Thesecharacteristics also make the material very well suited for the supportof small, i. e., less than about 2.0-2.5 nm, metal clusters orcrystallites. Thus the ceria-based mixed-metal oxide with thismorphology and bearing these small metal particles on its surface isideally suited for use as a catalyst in the WGS reaction where, it isreported, the CO chemisorbed on the surface of the metal particlesundergoes nucleophilic attack by oxide ions from the mixed metal oxide,converting it to CO₂ and reducing the mixed metal oxide which isreoxidized by the reaction of the oxide with water, a reaction thatliberates hydrogen.

Referring to the process by which supports having the aforementionedstructure and characteristics are formed, a novel method of synthesis byhomogeneous coprecipitation is used. While homogeneous coprecipitationmethods are known, including the use of urea as in the presentinvention, the steps and parameters of the process of the invention arespecific and unique, and yield the improved ceria-based mixed-metaloxide support in a novel and efficient manner. The synthesis method usedin the invention has the advantage of relatively short, or even no,aging, or maturing, time, the avoidance of expensive reagents likealcoxides, and the avoidance of super-critical solvent removal.

An important part of the support-forming method is that thecoprecipitation is performed in a very dilute metal salt solution, whichis believed to prevent particles/nuclei from growing to a larger size(i.e., >4 nm). The total metal concentration in the solution is lessthan 0.16 mol/L, is preferably less than about 0.02 mol/L, and mostpreferably, is less than about 0.016 mol/L. The solution, in addition tothe metal salt(s), also includes urea. Another important aspect is thatthe urea concentration must be high, at least 0.1 mol/L, and preferablybeing about 0.5-2 mol/L. The solution, containing the appropriateamounts of metallic salt and urea to attain the requisiteconcentrations, is heated to near boiling, which may include boiling,while stirring, to cause the hydrolysis of the urea and thus thereaction of the soluble metal ions with the urea hydroloysis products toform a cloudy suspension of nanocrystals. The coprecipitation of thevarious metal oxides quickly begins and is completed, typically in lessthan one minute. The resulting coprecipitate is gelatinous. Whilestirring is continued, the mixture of coprecipitate (hereinafterreferred to sometimes simply as “precipitate”) and solution mayoptionally be aged, or matured, though is not necessary for manyapplications. To the extent required at all, the step of aging thecoprecipitate mixture is relatively fast, being less than 72 hours,preferably less than about 24 hours, and most typically being in therange of 3 to 8 hours or less. The aging step comprises heating, ormaintaining the heating of, the mixture to, or near, its boilingtemperature for about, for example, 7 hours, and then continuing to stirand allowing to cool to ambient room temperature for an additionalperiod of, for example, about 16 hours. The continued heating after theformation of the nanocrystalline suspension is neither particularlyhelpful nor harmful, nor is an extended period of stirring during andafter cool down. Indeed, it has recently been discovered thatcrystallization may, and often does, occur when the gelatinouscoprecipitate is formed, thus further reducing or eliminating the needfor maturing the coprecipitate, particularly depending upon the useapplication of the material.

The mixture is then filtered, and the resulting filter cake is washed,typically twice, with de-ionized water at about boiling temperature.Importantly, the water associated with the filter cake is then replacedwith a water-miscible, low surface-tension solvent. This serves toreduce the capillary pressure exerted by the solvent on the solid oxideduring a subsequent drying step. The water-miscible, low surface-tensionsolvent may be an alcohol with 4 carbons or less, and preferably 3carbons or less, or a ketone or an ester, each with 4 carbons or less. Apreferred such solvent is dried 2-propanol, with other examplesincluding propanone (acetone), methyl ethyl ketone, and 1-propanol. Thismay be accomplished in various ways, but preferred herein is firstwashing the filter cake several times with the water-miscible, lowsurface-tension solvent at room temperature, and then mixing fresh,dried, water-miscible, low surface-tension solvent with the precipitateand heating to reflux for about 45 minutes. The need for the reflux washwill be determined by the effectiveness of the prior lower temperaturewashes in replacing the water. The washed precipitate may be freed fromexcess solvent by any of the several means known in the art includingfiltration, centrifugation, spray drying, etc. Alternatively, the washedprecipitate may be effectively suspended in a sufficient amount ofliquid and that suspension used either directly or after the addition ofa binder or binder components, to wash coat monoliths, foams, and/orother substrate objects. In a more concentrated form, the washedprecipitate may be extruded, as by a syringe or the like.

The resulting coating or extrudate then undergoes a drying step toremove the remaining solvent. This may be accomplished by any of thevariety of means known in the art, but vacuum oven drying at about 70°C. for about 3 hours is effective, and the extrudate may then remain inthe oven at that same temperature, but without vacuum, for an additionalperiod that may be about 16 hours.

Following drying, the oxide, or the aforementioned formed and driedmixed metal oxide may be calcined at 250° C.-600° C., and preferablyabout 350° C.-500° C., for an interval sufficient to remove adsorbedspecies and strengthen the structure against premature aging. Lowertemperatures typically mean more physisorbed and chemisorbed solventand/or carbonates, while higher temperatures and longer times mean thereverse. In an exemplary process, the calcining required is about 4hours with a heating rate of about 2° C./minute. The calcining processtypically begins at a temperature of about 70° C., and the calciningtemperature selected is based on a balance of increased surface area atthe lower end of the time/temperature range vs. assured removal ofcontaminants at the upper end.

Following calcination, the precipitate possesses the properties desiredof the support of the invention, to wit, homogeneous mixed oxides of atleast cerium and typically zirconium, hafnium, and/or variousconstituents, that are nano-crystalline, typically less than 4 nm forcalcinations at 500° C. or less, and that collectively define astructure having large pores, typically of more than 4 nm, in the rangeof more than 4 nm to less than about 9 nm, and thereby have a largesurface area greater than 150 m²/g, typically 180 m²/g or greater. Thiscombination of surface area and average pore diameter translates intorelatively low internal mass transfer resistance. However, if that valuebecomes too small because of excessive pore size and/or volume, theeffective number of sites per crystallite aggregate necessarilydecreases and the amount of effective surface area per unit reactorvolume also decreases. For a porous material of given density, D, theskeletal volume, V_(S), is 1/D, such that the total volume of a gram ofmaterial, V_(T), is the sum of the pore volume, V_(P), +skeletal volume,V_(S). From this, the surface area/gram/V_(T) yields the surface areaper unit volume of material, and it is this value which the inventionseeks to maximize. Accordingly, it has been determined that the surfacearea per unit volume of material should be greater than 320 m²/cm³, andpreferably greater than 420 m²/cm³. Viewed yet another way, it has beendetermined that the ratio of pore volume, V_(P), to the particle, orskeletal volume, V_(S), should not exceed about 2.5. It is important torealize that skeletal volume is more appropriate than mass when dealingwith a high skeletal or crystallite density like CeO₂, (7.132 g/cm³) .For example, a given surface area/g of CeO₂ will translate to about 2½times the surface area, m², per unit volume, cm³, than for the samegiven surface area/g of a the less-dense catalyst support material SiO₂,which has a crystallite density of 2.65 g/cm³.

At this point the mixed metal oxide, as described, is complete as to itschemical and micro-morphology, although if necessary itsmacro-morphology (> several microns) may be adjusted. If the mixed metaloxide is to be loaded with precursors of what will become a highlydispersed catalytically-active metallic phase, further treatment stepsmay be necessary as will be described later in connection with furtheraspects of the invention. Immediately following are examples in whichthe above-described process or some variants thereof, are described indetail for illustrative or comparative purposes. These examples are ofmixed metal oxides used as catalyst supports, and are intended to beillustrative, and not limiting.

It is helpful at this juncture to identify and/or describe thetechniques used to identify crystallite size, support surface area, porevolume, and average pore diameter herein.

Average crystallite size was determined by powder X-ray diffraction linebroadening using powder X-ray diffraction patterns (PXRD) and theDebye-Scherrer equation:t=(0.9*λ)/B Cos θ_(B), where:

-   -   t=Crystallite thickness;    -   0.9 or 1.0=crystal shape factor;    -   λ=Wave length of Radiation (angstroms);    -   B=breadth of diffraction peak (radians);    -   θ_(B)=Bragg angle (degree).

The surface area of the mixed metal oxide support was determined byfirst determining nitrogen adsorption-desorption isotherms at liquidnitrogen boiling temperature by the classical volumetric method with aMicromeretics ASAP 2010 instrument and then calculating the surface areausing the well-known BET method.

Pore volume was determined by the volume of the adsorbate taken at arelative pressure of P/P₀=0.98955093.

Pore size distribution data and curves were calculated from thedesorption branch of the isotherm using the BJH method.

The average pore diameter was determined by dividing the pore volume bythe surface area, the result being multiplied by a factor depending onthe pore shape. The equation 4V/A was used for cylindrical pores where Vand A are, respectively, the pore volume and the surface area asdetermined above.

EXAMPLE 1

The following is an example demonstrating the method of preparation fora ceria-zirconia nanocrystalline support material as described in thisinvention and the resulting properties. A Ce_(0.65)Zr_(0.35)O₂ catalystsupport (Sample UR27) is prepared by dissolving 26.7 g of(NH₄)₂Ce(NO₃)₆, 7.2 g of ZrO(NO₃)₂. xH₂O and 576 g of urea in 4800 mL ofde-ionized water. The solution is heated to its boiling temperaturewhile stirring until coprecipitation is observed. The mixture is thenaged at boiling temperature for 7 hours and then is left stirring atroom temperature for 16 hours. The mixture is filtered using a Büchnerfunnel. The resulting filter cake is washed twice with 500 mL ofde-ionized water at boiling temperature while stirring for 10 minutes,and then filtered again after each washing step. Then the filter cake iswashed three times with 100 mL of dried 2-propanol while in the Büchnerfunnel. Then, if necessary, the precipitate is mixed with 400 mL ofdried 2-propanol and heated to reflux for 45 minutes and then filteredagain before being extruded, as through a syringe. The extrudates aredried in a vacuum oven at 70° C. for 3 hours and then left in the ovenat 70° C. (overnight) for 16 hours without vacuum. The extrudates arethen calcined at 500° C. for 4 hours with a heating rate of 2° C./min.

After calcination at 500° C., the surface area of the support is 180m²/g and has an average crystallite size of 34.1 Å (3.41 nm). The porevolume is 0.25 cm³/g and the average pore diameter is 55 Å (5.5 nm).Another support (Sample UR 17) prepared with essentially the sameparameters has a surface area of 187 m²/g and an average crystallitesize of 32.3 Å (3.23 nm).

Comparative Example 2

The following is an example demonstrating how a ceria-zirconiananocrystalline material, prepared from a metal solution of lowerdilution compared to Example 1, yields a smaller surface area that isunsatisfactory. A Ce_(0.65)Zr_(0.35)O₂ catalyst support (Sample UR48)was synthesized as described in Example 1 with the modification that theamount of urea and water used was 60 g and 500 mL, respectively. Aftercalcination, the surface area of the support is 144 m²/g, the porevolume is 0.22 cm³/g, and the average pore diameter is 60 Å.

Comparative Example 3

The following is an example demonstrating a ceria-zirconia oxide with asignificantly reduced and undesirable surface area when water is notdisplaced or exchanged by a low surface-tension solvent as in Example 1.A Ce_(0.65)Zr_(0.35)O₂ catalyst support (Sample UR19) was synthesized asdescribed in Example 1 with the modification that there was noreplacement of water, as with the low surface tension solvent orotherwise. After calcinations at 500° C., the surface area of thesupport) was 116 m²/g.

EXAMPLE 4

The following is an example demonstrating a ternary mixed metal oxide inaccordance with the invention where the dopant is a rare-earth metal. ACe_(0.625)Zr_(0.325)PrO₂ catalyst support (Sample UR26) was prepared asdescribed in Example 1 with the modification that the amount of(NH₄)₂Ce(NO₃)₆ and ZrO(NO₃)₂ used was 25.7 g and 6.7 g, respectively,and 1.63 g of Pr(NO₃)₃.6H₂O was also added to the solution. Aftercalcination, the surface area of the support is 182 m²/g, the porevolume is 0.26 cm³/g, and the average pore diameter is 56 Å (5.6 nm).

EXAMPLE 5

The following is an example demonstrating yet another ternary mixedmetal oxide in accordance with the invention where the dopant is arare-earth metal. A Ce_(0.625)Zr_(0.325)La_(0.05)O₂ catalyst support(Sample UR37) was prepared as described in Example 4 with themodification that 1.62 g of La(NO₃)₃.6H₂O was used instead of the 1.63 gof Pr(NO₃)₃.6H₂O, and the resulting extrudates were calcined at 450° C.instead of 500° C. After calcination, the surface area of the support is204 m²/g, the pore volume is 0.25 cm³/g, and the average pore diameteris 49.9 Å (4.9 nm). The average crystallite size is 28.6 Å (2.86 nm).

EXAMPLE 6

The following is an example demonstrating a ternary mixed metal oxide inaccordance with the invention where the dopant is a non rare-earthmetal. A Ce_(0.7)Zr_(0.22)Nb_(0.08)O₂ catalyst support (Sample UR84) wasprepared as described in Example 1 with the modification that the amountof (NH₄)₂Ce(NO₃)₆ and ZrO(NO₃)₂ used was 28.8 g and 4.54 g,respectively, and 7.94 g of niobium oxalate was also added to thesolution. After calcination, the surface area of the support is 152m²/g, the pore volume is 0.24 cm³/g, and the average pore diameter is 64Å (6.4 nm).

EXAMPLE 7

The following is yet another example demonstrating a ternary mixed metaloxide in accordance with the invention where the dopant is a nonrare-earth metal. A Ce_(0.70)Zr_(0.20)Ga_(0.05)O₂ catalyst support(Sample UR74) was prepared as described in Example 1 with themodification that the amount of (NH₄)₂Ce(NO₃)₆ and ZrO(NO₃)₂ used was28.8 g and 4.13 g, respectively, and 2.13 g of Ga(NO₃)₃.xH₂O was alsoadded to the solution. After calcination, the surface area of thesupport is 180 m²/g, the pore volume is 0.24 cm³/g, and the average porediameter is 54 Å (5.4 nm).

EXAMPLE 8

The following is an example demonstrating the effect of a rare-earthdopant in combination with cerium, but in the absence of zirconium orhafnium. A Ce_(0.80)Gd_(0.20)O₂ catalyst support (Sample UR78) isprepared by dissolving 32.89 g of (NH₄)₂Ce(NO₃)₆, 6.77 g ofGd(NO₃)₃.6H₂O and 576 g of urea in 4800 mL of de-ionized water. Thecatalyst support is hereafter prepares as described in Example 5. Aftercalcination, the surface area of the support is 222 m²/g, the porevolume is 0.37 cm³/g, and the average pore diameter is 67 Å (6.7 nm). Inthis example, it is seen that the use of yet another rare earth dopant,in this instance Gd (gadolinium), in combination with cerium, but in theabsence of zirconium or hafnium, yields a particularly large surfacearea, pore volume on a unit weight basis, and pore diameter. However, onan activity-per-surface area basis, the performance of this catalystsupport was deficient, and reflects the likelihood that water gas shiftactivity is positively affected by the presence of Zr, and the reverseby its absence.

EXAMPLE 9

The following is an example demonstrating a ceria-hafnia nanocrystallinesupport material as described in this invention. A Ce_(0.65)Hf_(0.35)O₂catalyst support (Sample UR94) was synthesized as described in Example 1with the modification that 9.7 g of HfO(NO₃)₂.5H₂O was used instead of7.2 g of ZrO(NO₃)₂.xH₂O, and that the extrudates were calcined at 400°C. instead of 500° C. Following calcination, the surface area of thesupport was 180 m²/g, the pore volume was 0.21 cm³/g, and the averagepore diameter was 46.2 Å (4.62 nm). The average crystallite diameter was26.0 Å (2.6 nm). This oxide, after subsequent loading with Pt, wastested and found to be active for water gas shift catalysis.

In addition to the aforementioned Examples of numerous Samples, thefollowing Table 1 includes seven additional Samples of ternarymixed-oxides and their respective relevant properties: TABLE 1 Clcn SrfcAvg Avg Temp Area Pore Pore Vol Crys Sample Ce Zr Dopant Amount ° C.m²/g Dia Å cm³/g Size Å UR39 0.625 0.325 Sm 0.05 450 185 55 0.26 31 UR430.625 0.325 Eu 0.05 450 188 56 0.26 34 UR38 0.625 0.325 Gd 0.05 450 19654 0.26 32 UR46 0.625 0.325 Tb 0.05 450 202 53 0.27 33 UR47 0.625 0.325Yb 0.05 450 182 58 0.26 37 UR76 0.700 0.200 Mo 0.10 450 191 55 0.26 33UR70 0.750 0.200 Nb 0.05 450 175 65 0.28 38

The Samples of Table 1 each contain, on a metals-only basis, between 40and 85 atomic % of Ce, between 15 and 60 atomic % of Zr, and between 0and 10 atomic % of several different dopants. All of those Samples werecalcined at 450° C., and resulted in mixed metal oxides having surfaceareas of 175 to 202 m²/g, average pore diameters of 53 to 65 Å (5.3-6.5nm), pore volumes in the range of 0.25-0.28 cm³/g, and averagecrystallite sizes, as determined by the Jade program, of 31-38 Å. Thoughnot shown in Table 1, the lattice parameters of these samples were inthe range of 5.33 to 5.38 Å. The earlier-mentioned Samples of Examples1-9 serve to expand those ranges somewhat, with the lattice parameter,which is an indication of homogeneity, in particular expanding its rangeto 5.32-5.42 Å, the surface area increasing to 222 m²/g, the averagepore diameter range increasing to 50-67 Å, and the average crystallitesize range extending to 29 Å. The pore volumes of relevant Examples 1, 4and 5 were similar to those of Table 1; the Example 9, with Hf insteadof Zr, had a pore volume of 0.21 cm3/g; whereas the non-Zr, non-Hf,Example 8 had a pore volume of 0.37 cm³/g.

Consideration will now be given to the data presented in Table 2,wherein the identifiers UR86, UR87, and UR88 represent three mixed metaloxides of the invention, with differing amounts of Ce and Zr, and CZ68and CZ80 represent Ce and Zr oxides made in accordance with theteachings of the article by Terribile, et al referenced earlier in theBackground Art section. The numbers immediately following theidentifiers are the temperatures (° C.) at which they were calcined. TheCeO has a density of 7.132 and the ZrO₂ density is 5.6 g/cm³. From leftto right, beginning with the fourth column, there is surface area (SA);pore volume (V_(P)); Average Pore diameter; total volume (V_(T)), whichis the sum of V_(P) and V_(S); Surface Area per V_(T); Density of Ce/Zrskeleton (D); volume of skeleton (V_(S)); and the ratio of V_(P) toV_(S) (V_(P)/V_(S)). TABLE 2 V_(P) Ave V(t) D(s) SA cm3 Pore cm3 SA/Vtg/ % Ce % Zr m2 per dia of m2 cm3 Vs Vp/ # 7.132 5.6 gr gram nm gram cm3skel cm3/g Vs UR88-450 0.8 0.2 215 0.27 5.02 0.42 516 6.83 0.147 1.84UR88-550 0.8 0.2 175 0.24 5.49 0.39 453 6.83 0.147 1.64 UR88-650 0.8 0.2126 0.21 6.67 0.36 353 6.83 0.147 1.43 UR87-450 0.75 0.25 214 0.29 5.420.44 488 6.75 0.148 1.96 UR87-550 0.75 0.25 164 0.26 6.34 0.41 402 6.750.148 1.75 UR87-650 0.75 0.25 127 0.23 7.24 0.38 336 6.75 0.148 1.55UR86-450 0.68 0.32 197 0.27 5.48 0.42 468 6.64 0.151 1.79 UR86-550 0.680.32 156 0.25 6.41 0.40 389 6.64 0.151 1.66 UR86-650 0.68 0.32 116 0.227.59 0.37 313 6.64 0.151 1.46 CZ80-450 0.8 0.2 208 0.86 16.54 1.01 2076.83 0.147 5.87 CZ80-650 0.8 0.2 163 0.56 13.74 0.71 231 6.83 0.147 3.82CZ68-450 0.68 0.32 235 0.66 11.23 0.81 290 6.64 0.151 4.38 CZ68-650 0.680.32 170 0.42 9.88 0.57 298 6.64 0.151 2.79

A review of Table 2 shows a decrease in surface areas as a function ofcalcining temperature, with the more desirable surface areas being forcalcinations below about 500° C.; relatively smaller pore volumes forthe mixed oxides of the invention and relatively larger for the other;pore diameters less than about 8 or 9 nm for the material of theinvention and larger for the other; relatively smaller total volumes forthe invention and larger for the other; relatively larger surface areasper total volume (typically greater than 320-420 m²/cm³) for thematerials of the invention; and V_(P)/V_(S) ratios under 2.5 for thematerial of the invention.

It has been discovered that the surface area of the ceria-basedmixed-metal oxide material is, to some extent, a function of the gasatmosphere condition under which the material is calcined. A flowing gasis preferable to a static gas condition, air provides an economic sourceof a flowing gas that provides good results, and calcining the oxidematerial under flowing CO₂, or more preferably under CO₂ mixed with adilute O₂ mixture, such as a mixture ranging from 20% CO₂; 40% O₂; and40% Ar to 80% CO₂; 10% O₂; and 10% Ar, or even more preferably under CO₂followed by such a mixture of dilute O₂, but without CO₂, appears toyield particularly large surface areas, in excess of 250 m²/g.

EXAMPLE 10

The following example demonstrates how the ceria-zirconiananocrystalline support material described in this invention exhibits ahigher surface area when calcined under a flowing gas environment and aneven higher surface area when calcined under a CO₂ environment. ACe_(0.65)Zr_(0.35)O₂ catalyst support (Sample UR119) is prepared bydissolving 53.5 g of (NH₄)Ce(NO₃)₆, 14.2 g of ZrO(NO₃)₂.xH₂O, and 1152 gof urea in 9600 mL of de-ionized water. The solution is heated to itsboiling temperature while stirring until the co-precipitation isobserved. The mixture is then aged for 6 hours and then is left stirringat room temperature for 15 hours. The mixture is filtered using aBüchner funnel. The resulting filter cake is washed twice with 1000 mLof de-ionized water at boiling temperature while stirring for 10minutes, and then filtered again after each washing step. The filtercake is then mixed three times with 200 mL of anhydrous 2-propanol whilestill in the Büchner funnel. The filter cake is then mixed with 800 mLof anhydrous 2-propanol and heated to reflux for 45 minutes and thenfiltered again before being extruded through a syringe. The extrudatesare dried in a vacuum oven at 70° C. for 3 hours and then left in theoven at 70° C. overnight (15 hours) without vacuum. The extrudates arethen comminuted to less than 30 mesh size. The comminuted powder isseparated into 6 parts and calcined at 400° C. for 4 hours at a heatingrate of 10° C./min, each under varying gas environments. The gas flow isset at a rate of 2 L/min. The resulting physical properties are given inTable 3. TABLE 3 Surface Pore Pore Calcination Calcination Area VolumeDiameter Atmosphere Temp (° C.) (m²/g) (cm³/g) (Å) Static 400 182 0.2353 Air 400 230 0.24 42 N₂ 400 233 0.25 43 50% O₂/Ar 400 232 0.25 42  5%H₂/Ar 400 228 0.25 44 CO₂ 400 251 0.30 41

Compared to a static calcination, where no gas is flowing through thefurnace during heating, calcining in a flowing gas environment resultsin materials with higher surface areas. A 26% increase in surface areais observed for air, nitrogen, an oxygen-containing environment (50% O₂,50% Ar), and a hydrogen-containing environment (5% H₂, 95% Ar), from 180m²/g to ˜230 m²/g. Furthermore, when CO₂ is used, the surface area isincreased 38%, to ˜250 m²/g.

EXAMPLE 11

The following example demonstrates how the ceria-zirconiananocrystalline support material described in this invention exhibits ahigher surface area when calcined under a CO₂ environment combined withan oxygen-containing environment. A Ce_(0.65)Zr_(0.35)O₂ catalystsupport (Sample UR143) was synthesized as described in Example 10 withthe modification that the mixture was then aged for 7 hours instead of 6and the comminuted powder was separated into four parts instead of six.Of the four parts, each was calcined under a different gas ratio of CO₂and an oxygen-containing environment. The combined gas flow was set at arate of 2 L/min. The resulting physical properties are given in Table 4.TABLE 4 Surface Area Sample % CO₂ % O₂ blend* (m²/g) UR143 50 50 238UR143 60 40 244 UR143 75 25 242 UR143 90 10 240*The O₂ blend is 50% O₂/50% Ar

The surface areas achieved are within error of one another signifying nochange with varying gas ratio. The overall surface areas are stillhigher than that achieved for a static environment or for the othernon-CO₂ gas environments tested, but less than that achieved for a CO₂only environment (Example 10).

EXAMPLE 12

The following example demonstrates how the ceria-zirconiananocrystalline material described in this invention exhibits a highersurface area when calcined under a CO₂ environment followed by anoxygen-containing environment. The Ce_(0.65)Zr_(0.35)O₂ catalyst supportused for this example is the same catalyst prepared and described inExample 11. A portion of the sectioned comminuted powder is heated to400° C. at a heating rate of 2° C./min, under a CO₂ environment. Then,the furnace is left to isotherm for 4 hours at 400° C. also under a CO₂environment. After 4 hours, the gas is switched to that of anoxygen-containing environment, specifically 50% O₂, 50% Ar, and thefurnace is cooled naturally to 25° C. The gas flow throughout thiscalcination process is set at a rate of 2 L/min. After calcination, theresulting material has a surface area of 237 m²/g. This surface area iscomparable to that achieved in Example 11.

EXAMPLE 13

The following example demonstrates how the ceria-zirconiananocrystalline material described in this invention exhibits a highersurface area when the oxygen-containing environment in Example 12 ischanged to that of air. A Ce_(0.65)Zr_(0.35)O₂ catalyst support (UR137)is synthesized as described in Example 10 with the modification that themixture is aged for 8 hours instead of 6 hours. The comminuted powder isheated to 400° C. at a heating rate of 2° C./min, under a CO₂environment. Then, the furnace is left to isotherm for 4 hours at 400°C. also under a CO₂ environment. After 4 hours, the furnace is coolednaturally and at 265° C., the gas is switched to that of air, and thefurnace is further cooled to 25° C. The gas flow throughout thiscalcination process is set at a rate of 2 L/min. After calcination, theresulting material has a surface area of 232 m²/g. This value iscomparable to that achieved in Example 12 when an oxygen-containingenvironment is used for calcination.

EXAMPLE 14

The following example demonstrates how a higher surface area can beachieved also for a ceria-hafnia nanocrystalline material. ACe_(0.65)Hf_(0.35)O₂ catalyst support (Sample UR174) is synthesized asdescribed in Example 9 with the modification that 9.5 g ofHfO(NO₃)₂.xH₂O was used instead of 9.7 g, the mixture was aged for 6hours instead of 7 hours, and the dried extrudates were comminuted toless than 30 mesh size before calcination. The comminuted powder wascalcined at 380° C. for 2 hours at a heating rate of 10° C./min, under aCO₂ environment combined with an oxygen-containing environment. Theexact gas composition is 90% CO₂, 5% O₂, 5% Ar. The gas flow is set at arate of 2 L/min. After calcination, the surface area was 189 m²/g, porevolume 0.28 cm³/g and average pore diameter 60 Å (6.0 nm). Compared to asimilar support material containing zirconium instead of hafnium, as forexample Sample UR 179 (Ce_(0.65)Zr_(0.35)O₂) with a surface area valueof 242 m²/g, the surface area is 28% lower. However, when comparing twomaterials with very different densities (e.g. D(ZrO₂) is 5.6 g/cm3 whileD(HfO₂) is 9.7 g/cm³), it is instructive to compare instead the skeletaldensities. Sample UR179 (Ce_(0.65)Zr_(0.35)O₂) has a skeletal density of1596 m²/cm³, while the sample prepared in this example, Sample UR174(Ce_(0.65)Hf_(0.35)O₂) has a skeletal density of 1516 m²/cm³. Thedifference between skeletal densities is only 5 percent. Thus, Example14 shows the invention's applicability to high surface areaceria-zirconia nanocrystalline materials.

It has recently been discovered that crystallization may, and oftendoes, occur when the gelatinous coprecipitate is formed, thus furtherreducing or eliminating the need for maturing the coprecipitate,particularly depending upon the use application of the material. To theextent that powder X-ray diffraction analysis has revealed that theprecipitate gel exhibits a crystalline pattern matching closely that ofthe finished oxide and not the hydroxide that would have been expected,one is given evidence that the chemistry of this system is differentfrom what would otherwise be expected. Moreover, this characteristic hasimplications for distinct morphology and physical properties since thestructure of the material is set upon conception from the aqueous phaseand is therefore more robust during processing. This characteristicfurther enables the shortening, or even elimination, of the aging step,depending upon the end use application.

EXAMPLE 15

The following example demonstrates how the ceria-zirconiananocrystalline material described in this invention exhibitscrystalline properties immediately after precipitation of the precursorsinto the gel. A Ce_(0.625)Zr_(0.325)Sm_(0.05)O₂ catalyst support (SampleUR216b) is prepared by dissolving 51.4 g of (NH₄)Ce(NO₃)₆, 13.2 g ofHfO(NO₃)₂.xH₂O, and 576 g of urea in 4800 mL of de-ionized water. Thesolution is heated to its boiling temperature while stirring until theco-precipitation is observed. Immediately after the precipitate isformed, a sample is collected for analysis. The powder X-ray diffractionof the swollen gel is given in FIG. 1 and shows a crystalline patternmatching closely to that of Ce_(0.6)Zr_(0.4)O₂. The gelatinousprecipitate is therefore classified as a crystalline material. This iscontrary to reports in the literature which traditionally state that anamorphous metal hydroxide material is formed upon precipitation, whichis converted to a crystalline metal oxide structure only after thecalcination step. The presence and recognition of this feature is veryunique, and also has implications for distinct morphology and rarephysical properties since the structure of the material is set uponconception from the aqueous phase and is therefore more robust duringprocessing.

EXAMPLE 16

The following example demonstrates how the ceria-zirconiananocrystalline material described in this invention exhibits similarmorphological properties when aged for 15 minutes versus a 7-hour agingperiod. A Ce_(0.66)Zr_(0.35)O₂ catalyst support (Sample UR117) wasprepared as described in Example 10 up to the aging process. Sampleswere collected at various time increments during the aging process,filtered and placed in 2-propanol to exchange the water in the gel withalcohol. The suspensions were placed on specimen grid normally used inconjunction with a Transmission Electron Microscope (TEM) and thenanalyzed by a High Resolution Scanning Electron Microscopy (HR-SEM). Theimages taken for both a 15-minute and a 7-hour aging time are given inFIG. 2. The morphology in both cases is the same and therefore anymorphological evolution that takes place in solution must do so withinthe first 15 minutes after the co-precipitation.

EXAMPLE 17

The following example demonstrates how the ceria-zirconiananocrystalline material described in this invention exhibits a similarsurface area despite the amount of aging time. Various ceria-zirconiaand doped ceria-zirconia supports were synthesized and their aging timewas varied to help elucidate the effect of aging time on surface area.The syntheses were all similar to that described for Example 1 and allsupports were calcined between 400-450° C. for anywhere between 2 and 4hours with a heating rate of either 2 or 10° C./min. The results aregiven in Table 5. TABLE 5 Surface Area Sample Composition Aging Time(m²/g) UR209 Ce_(0.60)Zr_(0.40)O₂ 0 minutes 207 UR215Ce_(0.625)Zr_(0.325)La_(0.05)O₂ 10 minutes 203 UR179Ce_(0.65)Zr_(0.35)O₂ 4 hours 242 UR97 Ce_(0.65)Zr_(0.35)O₂ 7 hours 212UR136b Ce_(0.65)Zr_(0.35)O₂ 21 hours 229The La additive in UR215 does not affect significantly the surface areavalues obtained.

Despite the large difference in aging time, the surface areas achievedwere close to being within range of one another. In addition, there isno linear trend and so it is concluded that the aging time has no effecton the resulting surface area. A long aging time of 20 hours offers nobenefit over a 4 or 7 hour aging time. In fact, as shown by SampleUR209, for all practical purposes, the aging step can be removedentirely from the synthesis protocol.

For ceria-based mixed-metal oxides that are to be used as catalystsupports and which comprise cerium oxide and typically also zirconiumand/or hafnium oxide, it is generally desirable that the oxide possess acubic structure. The cubic structure is generally associated withgreater oxygen mobility, and therefore greater catalytic activity.Moreover, the zirconium and/or hafnium provide thermal stability, andthus contribute to the thermal stability and life of a catalyst. Inaccordance with an aspect of the invention, it has been determined, byRaman spectroscopy, for example, that the cerium-based oxide made inaccordance with the invention exhibits a cubic phase lattice for evenrelatively high concentrations of dopants such as zirconium and/orhafnium. In recognition and application of that determination, it hasbeen found that the composition of the bulk ceria-based mixed-metaloxide is, advantageously: cerium, between 40% and 85%; zirconium orhafnium, or mixtures thereof, between 15% and 60%; one or members of thegroup: Ti, Re, Nb, Ta, Mo, W, Rh, Sb, Bi, V, Mn, Co, Cu, Ga, Ca, Sr, andBa, between 0% and 10%; and one or more members of the group: Y, La, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, between 0% and 10%; andwhere all percentages are on a metals-only atomic basis. In accordancewith one aspect of the invention, the cerium is between about 40% and70%, and the zirconium or hafnium, or mixture thereof, is between about25% and 60% and most preferably is greater than about 45%. Under anotheraspect of the invention, the cerium may be 60% or more. Moreover, theceria-based mixed-metal oxides exhibit, via Raman spectroscopy, a cubicstructure to the effective exclusion of the tetragonal phase, over all,or at least most, of the compositional range of interest for the severalembodiments.

The Yashima et al. article suggested that above 20 percent zirconia, theoxygen anion lattice distorts into a tetragonal phase, while the ceriumand zirconium cations remain in a cubic lattice structure, creating anon-cubic, metastable, pseudo-tetragonal phase lattice. Traditionally,powder X-ray diffraction (PXRD) has been used to identify the structureand symmetry of such phases. However, in the case of ceria-zirconiaoxides with very small crystallite sizes (i.e., less than 3 nm), thePXRD signal exhibits broadened peaks. Additionally, the signal producedby the oxygen atoms, which is a function of atomic weight, is drownedout by the intense signal produced by the cerium and zirconium cations.Thus any tetragonal distortion, caused by the oxygen atoms shifting inthe lattice, goes unnoticed in a PXRD pattern and the resulting patternappears cubic. A typical PXRD pattern of a synthesized ceria-zirconiamaterial with crystallite sizes <3 nm, overlaid with ICDD-PDF files fitto both cubic and tetragonal symmetry, respectively is shown in FIG. 3.It is apparent from these spectra that a clear decision cannot be madeas to the symmetry of these materials. In such cases, Raman spectroscopyand X-ray absorption fine structure (EXAFS) can be employed to observesuch phase transitions.

EXAMPLE 18

The following example demonstrates how both the ceria-zirconia andceria-hafnia nanocrystalline materials described in this inventionexhibit a cubic structure up to 35% zirconium or hafnium, respectively.Using the synthetic protocol described in Example 1, a high surface area(187 m²/g), nanocrystalline (3.5 nm), large pore (6 nm), ceria-zirconiaoxide support is prepared containing 35 atomic percent zirconia (SampleUR68). The Raman spectra is given in FIG. 4. The center peak around 490cm⁻¹ is representative of the Ce—O bond in a cubic lattice and theshoulder peak around 630 cm⁻¹ is representative of a structure defectcaused by the introduction of Zr or Hf into the Ce lattice. The samebehavior was observed for a ceria-hafnia material of similarcomposition.

EXAMPLE 19

The following examples demonstrate how the ceria-zirconia andceria-hafnia nanocrystalline materials described in this inventionexhibit a cubic structure up to 50% zirconium. Various ceria-zirconiasupports were prepared using the same protocol described in Example 1(Sample UR32, UR33, UR88, and UR128) and are given here as examples toelucidate the effect of doping levels on structure. All supports werecalcined between 400-500° C. for anywhere between 2 and 4 hours with aheating rate of either 2 or 10° C./min. The Raman spectra for theceria-zirconia materials are shown in FIG. 5. The results are similar tothat given in Example 18. All spectra exhibit the cubic peak around 490cm⁻¹ and no peaks representative of the tetragonal phase are detected.The same behavior was observed for the ceria-hafnia materials describedin this invention.

EXAMPLE 20

The following example demonstrates how the ceria-zirconiananocrystalline materials described in this invention exhibit a cubicstructure up to 59 mol % zirconium. A Ce_(0.41)Zr_(0.59)O₂ supportmaterial (Sample UR158) was prepared as described in Example 1 with themodification that the amount of (NH₄)₂Ce(NO₃)₆ and ZrO(NO₃)₂.xH₂O was16.86 and 11.98 g respectively, and the calcination temperature was 400°C. instead of 450° C. The Raman spectra shown in FIG. 6 gives a cubicpeak around 490 cm⁻¹ and no peaks representation of the tetragonal phasewas detected.

EXAMPLE 21

This example demonstrates how the ceria-zirconia nanocrystallinematerial described in this invention exhibit a cubic structure at 35%zirconium when calcined at temperatures up to 800 C. Two ceria-zirconiasupports were prepared using the same protocol described in Example 1.The first material (Sample UR68) has the compositionCe_(0.65)Zr_(0.35)O₂. The second material (Sample UR86) has thecomposition Ce_(0.68)Zr_(0.32)O₂. These two compositions are consideredas sufficiently close that a comparison can be made between the data.Sample UR68 was calcined at 400° C. and 700° C., while Sample UR86 wascalcined at 450° C., 550° C., and 650° C. The Raman spectra for theceria-zirconia materials are shown along with the correspondingcrystallite sizes in FIG. 7. All spectra retained the cubic peak around490 cm⁻¹ and no peaks representative of the tetragonal phase aredetected.

EXAMPLE 22

The following example demonstrates how a ceria-zirconia nanocrystallinematerial described in this invention exhibits a fractal morphology thusleading to a minimization in internal mass transfer resistance. Withinthe context of a nanoscale material, a pattern formed within anaggregate that is regular and repeating at increasing magnitudes ofscale can be considered as fractal. Having a fractal structureeliminates the need to design a catalyst material with very large porestructures. If fractal, the material possesses larger, 100 to 200 nm andmicron-sized pores in the aggregate (inter-particle void space, asopposed to intra-particle void space described earlier as forming the4-9 nm average diameter pores) which provide enough open space for gasmolecules to diffuse in and react. By reducing the internal porediameter (i. e., the inter-particle void space) to a smaller size, theinternal surface area may be increased, leading to a larger number ofactive sites per volume and a thus a higher catalytic activity. FIG. 8shows an electron micrograph of the nanocrystalline ceria-zirconiamaterial (Sample UR117) as described in the invention, highlighting itsfractal morphology.

Attention is now given to the facet of the invention concerned withloading a highly dispersed catalyst metal on the mixed metal oxidesupport just described, including the associated process and theresulting product. Although the following discussion and examples willuse Pt (platinum) as the catalyst metal loaded on the support, it willbe understood that other metals, and particularly noble metals, arewell-suited alternatives to Pt, such as Pd, Rh, Ir, Ru, and Os, as wellas alloys or mixed metal clusters containing noble metal including Group1B metals and/or Re. Further attention will be given hereinafter to theadvantage of including Re with Pt. The highly dispersed platinum, thatis, Pt crystallites that are typically less than 3 nm in diameter,preferably less than 2.5 nm in diameter, and most preferably less than 2nm in diameter, as applied to the mixed-metal oxide support of theinvention and in the manner and formulation of the invention, has aconcentration in the range from about 0.1 to about 6.0% by weight, asmetal, based on the final weight of the dried oxide support. While “low”metal loadings below about 1.0 wt % may be acceptable or even desirablein some situations, such as catalyst use at temperatures above 400° C.,high loadings of about 4 to 6 wt % may be needed to obtain the desiredcatalytic activity and life time at lower temperatures, for instance200° C. Such loading of a catalyst, such as Pt, on a large surface area,ceria-based support results in a catalyst that is particularly effectiveand efficient for use in WGS and PROX reactions.

To load the highly dispersed platinum on the support, several steps areinvolved, two of which are particularly important to the invention.Those important steps include, firstly, surface treating the ceria-basedsupport and secondly, selecting an/the appropriate formulation forloading the support. A preliminary step, which may also be accomplishedas a final step in the formation of the support, involves forming theoxide into a suitable form. This form may be that of a fine powder (<200mesh), 50 to 100 mesh granules, extrudates, pellets, with or withoutadded binder extrusion aides, etc., or a wash-coat or other coating on aceramic or metallic monolith, foam or wire mesh, again with or without asuitable binder.

The 50-200 mesh, ceria-based mixed-metal oxide support particles,preparatory to being metal-loaded, undergo surface treatment by beingheated in an acid solution containing one or more acids from the groupconsisting of amino acids, hydroxy dicarboxylic acids, hydroxypolycarboxylic acids and keto carboxylic acids, of which citric acidfrom the hydroxy polycarboxylic acid group and malic acid from thehydroxy dicarboxylic acid group are preferred, with malic acid beingparticularly preferred. These acids are selected to provide a mildreaction and serve to react with the oxide surface, forming sites thatbind noble metal-containing cations such as [Pt(NH₃)₄]⁺². It is believedthat these sites are sufficiently separated from each other as to yield,after either calcinations, calcinations and reduction, or reduction,very high noble metal dispersion, that is noble metal particlestypically less than 2.5 nm. The support particles are heated in analcohol solution, typically of ethanol, containing a selected acid, suchas malic or citric acid, at about 50° C. for 2-3 hours. The supportparticles are then rinsed with ethanol until a pH greater than 4 isattained.

After the surface treatment rinse, the support particles are submergedor immersed in a suitable solution containing the catalyst metal, inthis instance a tetraamineplatinum (II) nitrate solution, ammoniahydroxide and propanol, for 2-3 hours at room temperature.Tetraamineplatinum (II) nitrate or analogous salts of other noble metalsare usually chosen because they provide sufficiently stable, solublenoble metal cations to react with the treated surface, are halogen orsulfur free, are available at a reasonable cost, and on furthertreatment such as calcination smoothly decompose leaving no unwantedresidue. Tetraamineplatimum (II) chloride, bromide, etc. would work butthey contain halogens. For some noble metals, like Pd, other ligandslike ethanolamine can be substituted for ammonia, and the resultingwater containing solution contains a mixture of complexes of the typeM(II) [(NH₃)_(4−(x+n)) (ethanolamine)_(x)(H₂O)_(n)], where x+n is equalto or less than 4. Care must also be taken that the noble metal saltchosen doesn't undergo either spontaneous or light induced redoxreactions causing the noble metal to come out of solution.

Thereafter, the support particles are filtered through a 10 μm membranefilter and vacuum dried overnight (about 16 hours). Finally, the driedand metal-loaded catalyst support is calcined at a temperature in therange of 350°-500° C. for about 3-4 hours at a heating rate of about 2°C./min, to provide the finished catalyst. The essence of this phase ofthe process is to convert the metal-loaded protocatalyst to a stableform, through some appropriate combination of drying, calcining and/orreduction.

Following are 5 examples (23-28) in which the above-described process,or some variants thereof, are described in detail for illustrative andcomparative purposes, however it will be understood that these examplesare not intended to be limiting.

EXAMPLE 23

The following is an example demonstrating the method for loading thenoble metal on to a ceria-zirconia nanocrystalline support material, asdescribed in this invention, and the resulting properties. ACe_(0.65)Zr_(0.35)O₂ catalyst support is prepared substantiallyidentically to that of Example 1 above, except that it is calcined at400° C., rather than at 450° C. One gram of the support, comminuted to a50-200 mesh size, is heated in 5 mL of ethanol solution containing 0.1g/mL of citric acid at 50° C. for 2 hours. The support is then rinsedwith ethanol until the pH is greater than (>) 4. After that rinse, thesupport is submerged or immersed in 3.8 g of tetraamineplatinum (II)nitrate solution for 2 hours at room temperature. The tetraamineplatinum(II) nitrate solution contains 1.01 wt % Pt, 1 wt % ammonia hydroxide,and 15 wt % 2-propanol. The support is then filtered through a 10 μmTeflon® membrane filter and vacuum dried overnight (about 16 hours) at70° C. The loaded support is then calcined at 450° C., a temperaturesomewhat greater than that for calcining the support material itself,for about 4 hours with a heating rate of about 2° C./min. ICP analysishas determined the platinum loading to be 2.4 wt %, and the surface areato be 170-180 m²/g.

A Ce_(0.65)Zr_(0.35)O₂ sample made similar to Example 1 was seen to havea surface area of 186 m²/g, an average pore size of 6.82 nm, crystallitesize of 3.5 nm, and pore volume of 0.32 cm³/g. This sample was loadedwith 1.8 wt % of Pt, dried, calcined, reduced, and passivated. It wasseen to be a very active water gas shift catalyst. A microtomed sectionof this catalyst was examined by high resolution transmission electronmicroscopy (TEM), and revealed a nanocrystalline, porous microstructureof randomly oriented grains. The TEM observations corroborated thecrystallite size and average pore size data, determined by powder x-raydiffraction and BET techniques respectively. No regions identifiable ascrystalline Pt were identified in either bright field or dark field TEMimages or in electron diffraction patterns. After Fourier transformimage processing was used to remove the Ce_(0.65)Zr_(0.35)O₂ {111} and{200} lattice fringes from the image, a few regions about 2 nm in sizewere evident, which had a lattice fringe spacing consistent with Pt{111}. The scarcity of such images, when compared to the known Ptcontent of this material suggested that the majority of Pt crystalliteshad to be less than 2.5 nm, and typically less than about 1.5-2.0 nm insize.

Comparative Example 24

The following is an example of Pt loading without surface treatment.5.59 g of a CeO₂ support (calcined at 400° C., with 50-200 mesh size) issubmerged in 21.6 mL of tetraamineplatinum (II) nitrate solution formore than 48 hours at room temperature. A vacuum is applied when thesupport is first added to the solution. The solution is stirred for thefirst 5-10 minutes, and is then stirred occasionally during the courseof submergence. The tetraamineplatinum (II) nitrate solution contains1.04 wt % Pt, 1 wt % ammonia hydroxide, and 15 wt % 2-propanol. Thesupport is filtered through a 10 μm Teflon membrane filter and vacuumdried at 70° C. overnight. The loaded support is then calcined at 450°C. for about 4 hours with a heating rate of 20° C./min. The platinumloading was analyzed by ICP and seen to be only 0.56 wt %, thus showingthat the platinum loading is very low without surface treatment.

EXAMPLE 25

The following is an example of a Pt loaded ternary ceria-basedmixed-metal oxide where the dopant is a rare-earth metal. ACe_(0.625)Zr_(0.325)Pr_(0.05)O₂ catalyst support (Sample UR35) wassynthesized as described in Example 4 with the modification that theextrudate is calcined at 400° C. instead of 500° C. 1 g of thatCe_(0.625)Zr_(0.325)Pr_(0.05)O₂ support (50-200 mesh size) is heated in5 mL of an ethanol solution containing 0.05 g/mL of citric acid at 50°C. for 3 hours. The support is then rinsed with ethanol until the pH isgreater than (>) 4. After that rinse, the support is submerged orimmersed in 3.8 g of a tetraamineplatinum (II) nitrate solution for 2hours at room temperature, with occasional stirring. Thetetraamineplatinum (II) nitrate solution contains 1.01 wt % Pt, 1 wt %ammonia hydroxide, and 15 wt % 2-propanol. The support is then filteredthrough a 10 μm Teflon® membrane filter and vacuum dried overnight(about 16 hours) at 70° C. The loaded support is then calcined at 450°C., a temperature somewhat greater than that for calcining the supportmaterial itself, for about 4 hours with a heating rate of about 2°C./min. The platinum loading was determined to be 2.46 wt %, and thecatalyst surface area is 193 m²/g. The oxide crystallite size wasdetermined by powder x-ray diffraction to be 3 nm. High platinumdispersion is indicated by powder x-ray diffraction failing to detectany Pt or PtO because of its small crystallite size and confirmed bychemisorption techniques.

Comparative Example 26

The following is an example of a Pt loaded ceria-zirconia support wherethe dopant is a rare-earth metal and the surface treatment is not used.5.8 g of a Ce_(0.625)Zr_(0.325)Pr_(0.05)O₂ catalyst support, preparedsubstantially identically to Example 4 above except that it was calcinedat 400° C. rather than 500° C., is submerged in 22.1 g oftetraamineplatinum nitrate solution for more than 48 hours at roomtemperature, with occasional stirring. The tetraamineplatinum (II)nitrate solution contains 1.07 wt % Pt, 1 wt % ammonia hydroxide, and 15wt % 2-propanol. The support is then filtered through a 10 μm Teflon®membrane filter and vacuum dried overnight (about 16 hours) at 70° C.The loaded support is then calcined at 450° C., a temperature somewhatgreater than that for calcining the support material itself, for about 4hours with a heating rate of about 2° C./min. Platinum loading wasdetermined to be 1.5 wt %. This example again indicates that without thesurface treatment, Pt loading continues to be low.

EXAMPLE 27

The following is yet another example of a Pt-loaded ternary ceria-basedmixed-metal oxide where the dopant is a rare-earth metal. Four g ofCe_(0.625)Zr_(0.325)Pr_(0.05)O₂ support (calcined at 400° C., with50-200 mesh size) is heated in 20 mL ethanol solution of 0.07 g/mL malicacid at 50 ° C. for 3 h. The support is then rinsed with ethanol untilpH>4. After rinse, the support was submerged in 10.27 g oftetraamineplatinum (II) nitrate solution for 2 h at room temperature.The solution was stirred occasionally during the course of 2 hours. Thetetraamineplatinum (II) nitrate solution contained 1.03 wt % Pt, 1 wt %ammonia hydroxide and 15 wt % 2-propanol. The support was filteredthrough a 10 μm teflon membrane filter and vacuum dried at 70° C.overnight. It was then calcined at 450° C. for 4 h with a heating rateof 2° C./min. Platinum loading is 2.35 wt %. High platinum dispersion isindicated by the absence of Pt or PtO being observed by X-raydiffraction due to its small crystallite size.

In accordance with an aspect of the catalyst loading process of theinvention, it has been found to be particularly desirable to carefullytailor the surface treating (etch) step to the acid being used and theparticular composition of the ceria-based mixed-metal oxide support, inorder to accomplish the desired etch without excessive degradation ofthe oxide support material. This is particularly desirable for obtainingaccurate, reliable and reproducible catalyst loadings on nanocrystallinecomplex mixed metal oxides with surface areas greater than 200 m²/g. Inthis regard, the nanocrystalline mixed metal oxide is first titratedwith the acid to be used in order to establish a titration curve havingan equivalence point at which the pH stabilizes despite the continuedaddition of the titrant acid. The titration curve, or a family of suchcurves for differing acids and/or support materials, is then used tooptimize the etch process by selecting the quantity of acid determinedby the equivalence point.

EXAMPLE 28

The following example demonstrates the invention's applicability forPlatinum loading a high surface area, high zirconia content, cubicphase, ceria-zirconia nanocrystalline material. A Ce_(0.50)Zr_(0.50)O₂catalyst support (Sample FF3B) was synthesized according to the methoddescribed in Example 1, providing for different compositions. Theresulting material, calcined at 380° C. in a CO₂/O₂ environment andgiving a surface area of 248 m²/g, was prepared for titration by adding0.5 g of the support, comminuted to mesh size less than 120, to 100 mLethanol. A solution of 0.08 M malic acid dissolved in ethanol was usedto titrate the catalyst support by adding intervals of 0.1 mL until theequivalence point is sufficiently achieved (until the pH does not changesignificantly with each addition of acid titrant). The titration curveis given in FIG. 9. The optimum amount was determined to be 2 mL/gsupport. Based on this finding, 2.51739 g of the catalyst support,comminuted to a 80-120 mesh size, was heated in 5 mL of a 0.08 M malicacid/ethanol solution (as determined by titration) at 50° C. for 15minutes. The catalyst support was then washed thoroughly with ethanol,until the pH is greater than 4. After the rinse, the catalyst is driedand immersed in 10.0819 g of a 0.97% Platinum solution by weight for 2hours at room temperature. The Platinum solution consisted of 1.14410 gtetraammineplatinum nitrate, 1% by weight ammonia hydroxide and 15% byweight isopropanol (the balance is de-ionized water). The support wasthen filtered through a 10 μm Teflon® membrane filter and vacuum-driedovernight (16 hours) at 70° C. ICP results confirmed a loading of 2.08%Platinum. Thus, Example 28 shows that with the optimized loadingprocedure, including titration, a 50/50% by weight ceria zirconiamaterial is precisely loaded with Platinum

EXAMPLE 29

The following example demonstrates the invention's applicability forloading a high surface area, cubic phase, ceria-zirconia nanocrystallinematerial prepared under optimized calcination conditions. ACe_(0.58)Zr_(0.42)O₂ catalyst support (sample UR201C) was synthesizedaccording to the method described in Example 1, providing for differentcompositions. The resulting material was calcined at 380° C. in a CO₂/O₂environment and gave a surface area of 218 m²/g. The calcined materialwas prepared for titration by adding 0.5 g of the support, comminuted tomesh size less than 120, to 100 mL ethanol. A solution of 0.08 M malicacid dissolved in ethanol was used to titrate the catalyst support byadding intervals of 0.1 mL until the equivalence point is sufficientlyachieved (until the pH does not change significantly with each additionof acid titrant). The titration curve is given in FIG. 10. The optimumamount was determined from this to be 1 mL/g support. Based on thisfinding, 1.1161 g of the catalyst support, comminuted to a 80-120 meshsize, was heated in 2 mL of a 0.08 M malic acid/ethanol solution at 50°C. for 15 minutes. The catalyst support was then washed thoroughly withethanol, until the pH is greater than 4. After the rinse, the catalystwas dried and immersed in 3.3538 g of a 1.03% Platinum solution byweight for 2 hours at room temperature. The Platinum solution consistedof 2.5896 g tetraammineplatinum nitrate, 1% by weight ammonia hydroxideand 15% by weight isopropanol (the balance is de-ionized water). Thesupport was then filtered through a 10 μm Teflon® membrane filter andvacuum-dried overnight (16 hours) at 70° C. The loaded catalyst was thencalcined at 380° C. in static air for 4 hours, with a heating ramp of 2°C./min. ICP results confirmed a loading of 2.09% Platinum.

Comparative Example 30

The following example demonstrates platinum loading of a ceria-zirconiasupport with identical composition to that in Example 28 but notfollowing the procedure described in this invention. 6.7092 g ofcatalyst support with a composition of Ce_(0.58)Zr_(0.42)O₂ (SampleUR120B) was synthesized according to the method described in Example 1,providing for different compositions. The resulting material, calcinedat 400° C. in static air and with a surface area of 270 m²/g, issubmerged in 33.5 mL of a 0.53 M malic acid/ethanol solution for 3 hoursat 50° C. The catalyst support is then washed thoroughly with ethanol,until the pH is greater than 4. After the rinse, the catalyst is driedand immersed in 26.4724 g of a 1.02% Platinum solution by weight for 2hours at room temperature. The platinum solution consists of 0.7685 gtetraammineplatinum nitrate, 1% by weight ammonia hydroxide and 15% byweight isopropanol (the balance is de-ionized water). The support isthen filtered through a 10 μm Teflon® membrane filter and vacuum-driedovernight (15 hours) at 70° C. The loaded catalyst was then calcined at400° C. in static air for 4 hours, with a heating ramp of 2° C./min. ICPresults confirmed a loading of 2.80% Platinum, thus showing that thePlatinum loading is very high without the optimized surface treatment,including titration.

EXAMPLE 31

This example demonstrates the invention's applicability for loading ahigh surface area, cubic phase, ceria-hafnia nanocrystalline material.Ce_(0.65)Hf_(0.35)O₂ catalyst support (Sample UR149) was prepared wassynthesized according to the method described in Example 9, providingfor different compositions. The resulting material, calcined at 400° C.in static air and giving a surface area of 170 m²/g, was prepared fortitration by adding 0.5 g of the support, comminuted to mesh size lessthan 120, to 100 mL ethanol. A solution of 0.08 M malic acid dissolvedin ethanol is used to titrate the catalyst support by adding intervalsof 0.1 mL until the equivalence point is sufficiently achieved (untilthe pH does not change significantly with each addition of acidtitrant). The titration curve is given in FIG. 11. The optimum amountwas determined to be 4 mL/g support. Based on this finding, 1.6928 g ofthe catalyst support, comminuted to a 80-120 mesh size, was heated in 6mL of a 0.08 M malic acid/ethanol solution (4 ml/g) at 50° C. for 15minutes. The catalyst support is then washed thoroughly with ethanol,until the pH is greater than 4. After the rinse, the catalyst is driedand immersed in 6.7859 g of a 0.92% Platinum solution by weight for 2hours at room temperature. The Platinum solution consists of 1.3022 gtetraammineplatinum nitrate, 1% by weight ammonia hydroxide and 15% byweight isopropanol (the balance is de-ionized water). The support isthen filtered through a 10 μm Teflon® membrane filter and vacuum-driedovernight (15 hours) at 70° C. The loaded catalyst was then calcined at380° C. in static air for 4 hours, with a heating ramp of 2° C./min. ICPresults confirmed a loading of 2.08% Platinum. Thus, following thesurface treatment described in this invention, precise loading isachieved for ceria systems that include Hafnium.

Comparative Example 32

The following example demonstrates Platinum loading of a support withidentical composition to that in Example 31 but not following theprocedure described in this invention. 5.2969 g of a catalyst support ofCe_(0.65)Hf_(0.35)O₂ composition (Sample UR94), prepared according toExample 29, comminuted to a 60-200 mesh size, calcined at 400° C. instatic air, and having a surface area of 180 m²/g was submerged in 10 mLof a 0.53 M malic acid/ethanol solution for 3 hours at 50° C. Thecatalyst support is then washed thoroughly with ethanol, until the pH isgreater than 4. After the rinse, the catalyst is dried and immersed in19.3650 g of a 1.07% Platinum solution by weight for 2 hours at roomtemperature. The Platinum solution consists of 1.1361 gtetraammineplatinum nitrate, 1% by weight ammonia hydroxide and 15% byweight isopropanol (the balance is de-ionized water). The support isthen filtered through a 10 μm Teflon® membrane filter and vacuum-driedovernight (15 hours) at 70° C. The loaded catalyst was then calcined at400° C. in static air for 4 hours, with a heating ramp of 2° C./min. ICPresults confirmed a loading of 2.75% Platinum, thus showing that thePlatinum loading is also very high for the ceria systems with hafniumwithout the optimized surface treatment, including titration.

EXAMPLE 33

The following example demonstrates the invention's applicability forloading Platinum at higher levels on a high surface area, cubic phase,ceria zirconia nanocrystalline material. A Ce_(0.58)Zr_(0.42)O₂ catalystsupport (Sample UR201E) was synthesized according to the methoddescribed in Example 1, providing for different compositions. Theresulting material, calcined at 380° C. in a CO₂/O₂ environment, andwith a surface area of 218 m²/g, was prepared for titration by adding0.5 g of the support, comminuted to mesh size less than 120, to 100 mLethanol. A solution of 0.08 M malic acid dissolved in ethanol was usedto titrate the catalyst support by adding intervals of 0.1 mL until theequivalence point is sufficiently achieved (until the pH does not changesignificantly with each addition of acid titrant). The titration curveis given in FIG. 12. The optimum amount was determined to be 2 mL/gsupport. Based on this finding, 1.1111 g of the catalyst support,comminuted to a 80-120 mesh size, was heated in 2 mL of a 0.08 M malicacid/ethanol solution at 50° C. for 15 minutes. The catalyst support wasthen washed thoroughly with ethanol, until the pH is greater than 4.After the rinse, the catalyst was dried and immersed in 10.0165 g of a1.03% Platinum solution by weight for 2 hours at room temperature. ThePlatinum solution consisted of 2.5896 g tetraammineplatinum nitrate, 1%by weight ammonia hydroxide and 15% by weight isopropanol (the balanceis de-ionized water). The support was then filtered through a 10 μmTeflon® membrane filter and vacuum-dried overnight (16 hours) at 70° C.The loaded catalyst was then calcined at 380° C. in static air for 4hours, with a heating ramp of 2° C./min. ICP results confirmed a loadingof 4.14% Platinum, thus showing the invention is applicable to highPlatinum loadings.

EXAMPLE 34

The following example demonstrates the invention's applicability forloading platinum at lower levels on a high surface area, cubic phase,ceria zirconia nanocrystalline material. A Ce_(0.58)Zr_(0.42)O₂ catalystsupport (Sample UR201A) was synthesized according to the methoddescribed in Example 1, providing for different compositions. Theresulting material, calcined at 380° C. in a CO₂/O₂ environment and witha surface area of 218 m²/g, was prepared for titration by adding 0.5 gof the support, comminuted to mesh size less than 120, to 100 mLethanol. A solution of 0.08 M malic acid dissolved in ethanol was usedto titrate the catalyst support by adding intervals of 0.1 mL until theequivalence point is sufficiently achieved (until the pH does not changesignificantly with each addition of acid titrant). The titration curveis given in FIG. 13. The optimum amount was determined to be 2 mL/gsupport. Based on this finding, 1.0831 g of the catalyst support,comminuted to a 80-120 mesh size, was heated in 2 mL of a 0.08 M malicacid/ethanol solution at 50° C. for 15 minutes. The catalyst support wasthen washed thoroughly with ethanol, until the pH is greater than 4.After the rinse, the catalyst is dried and immersed in 1.1124 g of a1.03% Platinum solution by weight for 2 hours at room temperature. ThePlatinum solution consisted of 2.5896 g tetraammineplatinum nitrate, 1%by weight ammonia hydroxide and 15% by weight isopropanol (the balanceis de-ionized water). The support was then filtered through a 10 μmTeflon® membrane filter and vacuum-dried overnight (16 hours) at 70° C.The loaded catalyst was then calcined at 380° C. in static air for 4hours, with a heating ramp of 2° C./min. ICP results confirmed a loadingof 0.87% Platinum, thus showing the invention is applicable to lowPlatinum loadings.

EXAMPLE 35

The following example demonstrates the invention's applicability forloading Palladium at lower levels on a high surface area, cubic phase,ceria zirconia nanocrystalline material. A Ce_(0.58)Zr_(0.42)O₂ catalystsupport (Sample UR201J) was synthesized according to the methoddescribed in Example 1, providing for different compositions. Theresulting material, calcined at 380° C. in a CO₂/O₂ environment andhaving a surface area of 218 m²/g, was prepared for titration by adding0.5 g of the support, comminuted to mesh size less than 120, to 100 mLethanol. A solution of 0.08 M malic acid dissolved in ethanol was usedto titrate the catalyst support by adding intervals of 0.1 mL until theequivalence point is sufficiently achieved (until the pH does not changesignificantly with each addition of acid titrant). The titration curveis given in FIG. 14. The optimum amount was determined to be 2 mL/gsupport. Based on this finding, 1.0951 g of the catalyst support,comminuted to a 80-120 mesh size, was heated in 2 mL of a 0.08 M malicacid/ethanol solution (2 ml/g) at 50° C. for 15 minutes. The catalystsupport was then washed thoroughly with ethanol, until the pH is greaterthan 4. After the rinse, the catalyst was dried and immersed in 10.3980g of a 0.45% Palladium solution by weight for 2 hours at roomtemperature. The Palladium solution consists of 5.0600 g of a 10% byweight solution of tetraamminepalladium nitrate, 1% by weight ammoniahydroxide and 15% by weight isopropanol (the balance is de-ionizedwater). The support is then filtered through a 10 μm Teflon® membranefilter and vacuum-dried overnight (18 hours) at 70° C. The loadedcatalyst was then calcined at 380° C. in static air for 4 hours, with aheating ramp of 2° C./min. ICP results confirmed a loading of 2.09%Palladium, thus showing that the invention is applicable for achievingprecise Palladium loadings.

EXAMPLE 36

The following example demonstrates the invention's applicability forloading platinum on a high surface area, cubic phase, molybdenum-dopedceria zirconia nanocrystalline material ACe_(0.585)Zr_(0.315)Mo_(0.10)O₂ catalyst support (Sample UR202) wassynthesized according to the method described in Example 6. Theresulting material, calcined at 400° C. in static air and with a surfacearea of 212 m²/g, was prepared for titration by adding 0.5 g of thesupport, comminuted to mesh size less than 120, to 100 mL ethanol. Asolution of 0.08 M malic acid dissolved in ethanol is used to titratethe catalyst support by adding intervals of 0.1 mL until the equivalencepoint is sufficiently achieved (until the pH does not changesignificantly with each addition of acid titrant). The titration curveis given in FIG. 15. The optimum amount was determined to be 2 mL/gsupport. Based on this finding, 1.7875 g of the catalyst support,comminuted to a 80-120 mesh size, is heated in 3.5 mL of a 0.08 M malicacid/ethanol solution (2 ml/g) at 50° C. for 15 minutes. The catalystsupport is then washed thoroughly with ethanol, until the pH is greaterthan 4. After the rinse, the catalyst is dried and immersed in 3.3605 ofa 1.33% Platinum solution by weight for 2 hours at room temperature. ThePlatinum solution consists of 2.5896 g tetraammineplatinum nitrate, 1%by weight ammonia hydroxide and 15% by weight isopropanol (the balanceis de-ionized water). The support is then filtered through a 10 μmTeflon® membrane filter and vacuum-dried overnight (15 hours) at 70° C.The loaded catalyst was then calcined at 380° C. in static air for 4hours, with a heating ramp of 2° C./min. ICP results confirmed a loadingof 2.18% Platinum, thus showing that molybdenum-doped ceria-zirconiananopowders can achieve precise platinum loadings implementing thisinvention.

Comparative Example 37

The following example demonstrates platinum loading of a support withidentical composition to that in Example 34 but not following theprocedure described in this invention. 4.4586 g of aCe₀₇₀Zr_(0.20)Mo_(0.10)O₂ catalyst support (UR76) was synthesizedaccording to the method described in Example 6, providing for differentcompositions. The resulting material, comminuted to a 60-200 mesh size,calcined at 400° C. in static air, and with a surface area of 191 m²/g,was submerged in 22.5 mL of a 0.53 M malic acid/ethanol solution for 3hours at 50° C. The catalyst support was then washed thoroughly withethanol, until the pH was greater than 4. After the rinse, the catalystwas dried and immersed in 17.3292 g of a 1.04% Platinum solution byweight for 2 hours at room temperature. The Platinum solution consistedof 1.4513 g tetraammineplatinum nitrate, 1% by weight ammonia hydroxideand 15% by weight isopropanol (the balance is de-ionized water). Thesupport was then filtered through a 10 μm Teflon® membrane filter andvacuum-dried overnight (15 hours) at 70° C. The loaded catalyst was thencalcined at 400° C. in static air for 4 hours, with a heating ramp of 2°C./min. ICP results confirmed a loading of 2.97% Platinum, thus showingthat the optimized surface treatment is needed to achieve precisePlatinum loadings.

Although the catalytic activity afforded by Pt is relatively high andeffective for many processes, it has been discovered that the additionof rhenium (Re) with the loading of the noble metal (e.g., Pt) on themixed-metal oxide support yields a water gas shift and/or PROX catalystof particularly high activity. The turnover rate (TOR—the rate persecond at which Moles of CO are converted per Mole of Pt) issignificantly greater for such catalysts that include Re relative tothose that have Pt without Re. The Re is loaded, to a concentration inthe range of 0.5 to 6.0 wt %, on the mixed metal oxide supportpreviously loaded with the catalyst noble metal.

An aspect of the invention provides a preferred process for loading theRe on to the noble metal-loaded mixed-metal oxide. The source of the Reis not particularly critical, and may include ammonium perrhenate(NH₄ReO₄), perrhenic acid (HReO₄), rhenium carbonyl (Re₂(CO)₁₀), or thelike, with either of the first two mentioned examples having a costadvantage. The noble metal-loaded nanocrystalline mixed-metal oxide ofthe invention is immersed in an appropriate solvent; water or a watercontaining mixture, is an excellent solvent for the ammonium perrhenate(NH₄ReO₄) or perrhenic acid (HReO₄), while an organic solvent liketetrahydrofuran is an excellent solvent for rhenium carbonyl (Re₂(CO)₁₀)in this application. After an optional degassing or inert gas purgingstep, the noble metal-loaded, preferably Pt-loaded, nanocrystallinemixed metal oxide is contacted with a hydrogen containing gas to reduceand/or remove chemisorbed oxygen from the surface of the noble metal.Separately, the Re source material in the amount sufficient to add thedesired amount of Re to the noble metal-loaded nanocrystallinemixed-metal oxide is combined with the solvent to form a solution. Thissolution then replaces, or is added to, the solvent contacting the solidsuch that the noble metal-loaded mixed-metal oxide is contacted with theRe source-containing solution. Contact with the hydrogen-containing gasis continued to reduce the perrhenate ion, which in turn results in aclose association of the Re with the Pt. If rhenium carbonyl is used,the interaction with the noble metal under hydrogen is believed toresult in the decomposition of the rhenium carbonyl, thus depositing Reon the noble metal. As one skilled in the art will recognize, therhenium carbonyl can be replaced with another reasonably labile rheniumcompound/complex or an organometallic rhenium compound free of known orsuspected elements deleterious to the catalyst. The mixture is stirredunder the H₂ flow for a period, followed by a switch to an inert gas.After the hydrogen gas is substantially removed, oxygen or air may begradually introduced to the inert gas with care being taken that thetemperature is maintained below 50° C., preferably below about 30° C. Itis also preferable to remove all, or nearly all, of any flammablesolvent before the oxygen is introduced. This passivation step isimportant to prevent pyrophoric ignition upon contact with air, and maybe accomplished using alternative equivalent passivation techniques.

EXAMPLE 38

The following is an example demonstrating the method for loading rhenium(Re) on to ceria-zirconia nanocrystalline support material alreadyloaded with platinum, as described in this invention. A known amount ofa very high surface area, large pore ceria-zirconia mixed-metal oxide,previously loaded with a nominal 2 weight percent of highly dispersedplatinum as according to the invention, is diluted in a solution ofdegassed tetrahydrofuran (THF). This slurry is then bubbled with adilute hydrogen stream (3% in argon) until the platinum is sufficientlyreduced. With the system under a positive flow of a nominal 3% H₂ stream(in Argon), a solution of rhenium carbonyl in degassed THF is added viaa needle syringe. The mixture is then stirred at room temperature, stillunder a positive H₂ flow, until the THF evaporates. After the THF hascompletely evolved, the gas is switched to a dilute oxygen stream (5% inArgon) to (a) drive off the excess THF, and (b) to passivate the surfaceof the solid, thereby preventing any pyrophoric ignition upon contactwith air. This or an equivalent passivation step is necessary forsuccess. Mild heat may be applied to aid in the driving off of the THF.The dilute oxygen gas is left to flow over the powder until it reachesdry, yet pasty consistency. If necessary, at this stage the sample isthen placed in a vacuum oven at 70° C. to dry the powder to completion.

EXAMPLE 39

The following is an example demonstrating how the platinum loadedceria-zirconia nanocrystalline material, when prepared according to theinvention, undergoes a low temperature reduction due to the smallcrystallite size of the ceria-zirconia oxide support particles and thehigh dispersion of the platinum on these particles. ACe_(0.58)Zr_(0.42)O₂ catalyst support (Sample FF4) was preparedaccording to Example 1 providing for different composition. Afterpreparation, the support was calcined at 400° C. under static conditionsfor 4 hours with a heating rate of 10° C./min. The resulting surfacearea of the support is 178 m²/g and the average crystallite size is 34.3Å (3.43 nm). After calcination, the support was comminuted to 8-120mesh, loaded with a nominal 2 wt % platinum, and calcined again at 400°C. for 4 hours at a heating rate of 2° C./min. The loaded support,hereafter being referred to as the catalyst, was then placed in aTemperature Programmed Reduction (TPR) unit and heated to 400° C. whilebeing exposed to a gas mixture of 10% H₂ in nitrogen. The data profileis given in FIG. 16. The reduction peak occurs at 71° C. For micronscale materials, this reduction according to the literature is a surfacephenomenon, whereas for the nano-scale materials described here, thisreduction encompasses the whole mass. Hence, the nano-scale materialsdescribed in the invention reduce at an unexpectedly low temperature,giving implications for increased ease of reducibility and higherreactivity under operating water-gas shift conditions. Thus, thisexample shows the impact of nano-scale crystallite sizes on thereducibility of the ceria-zirconia materials.

Attention is now turned to a consideration of the catalytic activity ofthe metal-loaded, ceria-based mixed-metal oxide in water gas shiftreactions. As discussed previously, the noble-metal loaded, mixed-metaloxide supported catalyst of the invention is particularly suited for usein water gas shift reactions in hydrocarbon fuel processing systems. Inthat regard, Table 6 below serves to illustrate the effectiveness, oractivity, of a platinum-loaded, ceria-zirconia oxide supported catalyst,formulated and/or made in accordance with the invention, in convertingCO and H₂O to CO₂ and H₂. Table 6 conveys the level of thateffectiveness or activity, by tracking, as a function of temperature andtime, the quantity (%) of CO entering the catalytic reactor (nominally aconstant in the tests) vs. the quantity (%) of CO leaving the reactor.The difference is a measure of the conversion activity, eitherabsolutely or as a percentage, in converting CO (and H₂O) to CO₂ and H₂in the water gas shift reaction. Catalyst activity is given as(Micromoles of CO per second) per gram of catalyst.

It has been discovered that for a given reformate stream that is amixture of CO, CO₂, H₂, H₂O and other gases, where the H₂O/CO ratio isless than about 6, that a series of noble metal-loaded nanocrystallinemixed metal oxides have similar CO conversion to CO₂ turnover rates overa range of cerium to zirconium or hafnium ratios, but that surprisinglyfor H₂O to CO ratios between 6-30, keeping all other reactant andproduct concentrations fixed, differences in WGS activity emerge withdifferences in cerium-to-zirconium or hafnium ratios. Thus, it has beendiscovered that the WGS catalyst composition may be “tailored” tomaximize its activity for a particular range of feed gas composition,and likewise, that the feed gas composition may be tailored through theaddition of water and/or the removal of CO₂ and/or H₂ to operate in theregime of maximum catalyst activity. Furthermore, this aspect of theinvention leads to the use of a catalyst bed where either thecerium-to-zirconium or hafnium ratio changes or the Pt-to-Re ratiochanges, or both, along the catalyst bed to optimize performance. Inthis regard, feed gas, or reformate, compositions may be determined forselected regions along the flow of reformate across a catalyst bed in aWGS reactor, and a respective catalyst composition may then bedetermined and used for each respective region of the catalyst bed tooptimize performance across the bed as a whole.

EXAMPLE 40

The following is an example describing the activity test material,method and apparatus in greater detail. A mixed ceria zirconia oxide wasprepared according to this invention. The extrudate was dried andcalcined at 400 C. The resulting solid contained 65 atomic % Ce and 35atomic % Zr on a metals only basis. The solid had a surface areameasured by the BET method of 187 m²/g, with a pore volume of 0.29cm3/g. Thus its average pore diameter was 6.13 nm. Its averagecrystallite size was 3.4 nm by PXRD. Using a skeletal density of 6.6g/cm³, its surface area per cm³ skeletal volume was 425 m²/cm³, and itsratio of pore volume, V_(P), to skeletal volume, V_(S), i. e.,V_(P)/V_(S), is 1.93. The extrudate was crushed and sieved to yield aporous granular solid that passed through an 80 mesh sieve and wasretained on a 120 mesh sieve. This material was.heated in an ethanolicsolution of malic acid of 0.07 g/ml, to 50° C. for 3 hours, cooled, andrinsed with ethanol until the pH was >4. After rinsing, the solid wasimmersed in a 1% tetraammineplatinum (II) nitrate solution of 1%ammonium hydroxide and 15% 2-propanol for 2 hrs. The solid wasrecovered, dried under vacuum at 70° C. overnight. It was then calcinedat 450° C. for 4 hours. The platinum uptake from the tetraammineplatinum(II) nitrate solution is consistent with a platinum loading of 2.34 wt%. Then 0.5704 grams, or 0.50 cm³ by volume, of the 80-120 mesh catalystloaded with Pt, was uniformly blended with 5.00 cm³, or 9.4395 grams, of+40 mesh Strem Chemical alpha alumina granules and charged into a 0.50″O.D. 316L Stainless Steel reactor tube with 0.049″ walls with a 0.402″I.D. equipped with a 0.125″ O.D. Axial Thermowell. The net crosssectional area of the reactor was 0.74 cm². The catalyst charge wasseparated from the bottom frit by a 5.25″ length of 10 mesh alundumgranules and a thin wad of borosilicate glass wool. The catalyst and 40mesh alumina diluent bed together was 3.0″ long, and topped with a thinwad of borosilicate glass, above which was loaded about 5″ more of 10mesh alundum granules. As this was a down flow reactor, this 5″ topsection served to preheat the reaction gas mixture to reactiontemperature before it contacted the dilute catalyst. This was confirmedduring the initial heating and reduction by the internal, 0.0625″ K typethermocouple in the internal, axial thermowell. The 0.5″ O.D. reactortube was placed inside a tight fitting aluminum block to minimize axialtemperature gradients, and no axial thermal gradient was found at 320°C. under 20% hydrogen, 80% nitrogen flowing at 2.58 standard liters perminute (SLM). The catalyst, after loading into the reactor tube and thereactor tube secured into the reaction system, was first freed of anyadsorbed moisture by heating to 150° C. under high purity nitrogenflowing at 1.46 SLM, then the flow rate was further increase to about2.15 SLM with the addition of 8% very high purity hydrogen. After 1minute under 8% hydrogen the temperature was increased step-wise to 240°C. and held for 5 minutes, then increased to 290° C. and held for 5minutes, then increased to 330° C. and held for 5 minutes. Then thehydrogen concentration was increased to 15%, the flow to 2.2 SLM, andafter a 10-minute hold, the H₂ was increased further to 30% and the flowto 2.5 SLM. After 10 minutes at 30% H₂, and 330° C., the hydrogenconcentration was increased to 40% and these conditions were held for 15minutes. Then the temperature was adjusted to 320° C., the flow to 2.58SLM and the gas composition to 33% H₂O, 33% H₂, 5% CO₂ and the balanceN₂. These prerun conditions were held for 10 minutes before the carbonmonoxide was introduced. The gas mixture was then adjusted to about 4.9%CO, 33% H₂O, 30.4% H₂, 10.4% CO₂ and the balance N₂. The feed gascomposition was sampled every 20 minutes for about 60 minutes by meansof a gas chromatograph. The resultant N₂, CO₂ and CO values were thencompared to the values obtained at varying temperatures during a 320° C.to 200° C. stepwise down-ramp, during which the aluminum blocktemperature was held constant (within +/−2° C.) while product gassampling was conducted at each temperature. When the feed gascomposition was changed, as in the interval between 24 hours and 66hours when the quantity of CO in the feed was changed from 4.9% to 1.5%and back, the feed gas was sampled again to insure that conversioncalculations and rate calculations were accurate.

UHP grade compressed gases used in the experiments, N₂, Ar, H₂, CO andCO₂, were combined at ambient conditions in a mixing manifold.De-ionized water was fed to a steam generator that operates atapproximately 270° C. The generator was used to produce the steam thatwas mixed with the other gases (in the steam generator) and sent to thereactors. The composition of the final gas mixture was controlled byBrooks meters for the appropriate gases and a Porter Instruments watermeter for the steam flow. The inlet gases into the reactor and the exitgases from the reactor were analyzed using a FID and two TCDs.

In Table 6 below, “Effct T” refers to the effective catalyst temperaturecalculated as Effct T={[(2×Maximum Catalyst Bed Temperature, °C.)+Catalyst Exit Temperature, ° C.]/3} as measured by the internalthermocouple. The catalyst exit temperature in all cases wasapproximately the average of the catalyst inlet temperature and thealuminum block temperature. The “Feed” and “Exit” values for only CO aredepicted in Table 6 for conciseness. The Time in hours is calculatedfrom when the catalyst bed heater and initial N₂ flow was started. Onlyvery slight deactivation is seen after about the first 24 hours. TABLE 6Rate/ Feed Sec Effct Exit MircoMol

Hours T C % CO CO % gram 6.86 329 4.9 0.57 101.6 8.00 318 4.9 0.51 103.19.18 307 4.9 0.44 104.7 10.33 296 4.9 0.49 103.6 11.49 285 4.9 0.69 98.912.65 274 4.9 1.30 84.4 13.86 264 4.9 2.06 66.9 15.05 253 4.9 2.69 52.116.33 242 4.9 3.30 37.9 17.57 232 4.9 3.75 27.3 18.80 222 4.9 4.11 18.720.03 212 4.9 4.36 12.9 21.27 202 4.9 4.54 8.8 24.02 307 4.9 0.46 104.233.77 252 1.5 0.05 33.8 35.64 243 1.5 0.08 33.1 36.85 232 1.5 0.24 29.538.07 223 1.5 0.46 24.2 39.33 213 1.5 0.73 17.9 40.58 203 1.5 0.95 12.866.15 233 4.9 3.96 22.4 67.63 306 4.9 0.54 102.3 82.78 306 4.9 0.55102.3 109.89 306 4.9 0.60 100.9 112.00 274 4.9 1.76 74.0 141.99 305 4.90.62 100.4 143.32 273 4.9 2.02 67.7 161.03 287 4.9 0.54 102.3 162.25 3044.9 0.65 99.7 164.37 273 4.9 2.03 67.4 165.52 231 4.9 4.02 21.0

It will be noted from Table 6 that the catalyst is relatively moreactive at the higher temperatures over the practical range of operatingtemperatures between about 200° C. and 320° C., as would be anticipated.Moreover, for a feed of 1.5% CO, a relatively greater proportion (%) ofthe CO is removed at lower temperatures than for the 4.9% feed. Also ofimportance, a significant degree of catalyst stability is indicated bythe fact that its activity at 230° C. at 165 hours is at least 90% ofits activity at that temperature at 60 hours.

EXAMPLE 41

The following example demonstrates an optimized Ce:Zr ratio and varyingwater gas shift catalytic activities under different gas conditions.Table 7 compares catalysts UR129, UR176A1, FF4, and UR68, and it is seenthat sample FF4, which has a 58:42 Ce:Zr ratio, provides the best TurnOver Rate (TOR) performance under the second set of conditions (TOR²) .The optimal Ce/Zr ratio, as reflected by the data in Table 7, istherefore seen to be in the region of 50:50, with good results beingseen within the range from 60:40 to 40:60. TABLE 7 Impact of operatingconditions on water gas shift catalytic activity at 240° C. Catalyst CeZr Pt wt % TOR¹/s⁻¹ TOR²/s⁻¹ UR129 25 75 1.91 0.12 0.17 UR176A1 50 502.25 0.16 0.20 FF4 58 42 2.20 0.16 0.25 UR68 65 35 1.68 0.14 0.15¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂²1.5% CO, 45% H₂O, 25% H₂, 5% CO₂

EXAMPLE 42

The following example demonstrates the water gas shift catalyticactivity of two ceria-hafnia catalysts. Sample UR174 is a Pt loadedCe_(0.65)Hf_(0.35)O₂ catalyst prepared using the method described inExample 9. Catalyst UR149 is Pt loaded Ce_(0.65)Hf_(0.35)O₂ catalystwhere a different batch of HfO(NO₃)₂.5H₂O was used in the synthesis.Table 8 compares Catalysts UR174, UR149 and UR68 and it is observed thatHf is a better dopant under the second set of conditions but that theperformance of the ceria-hafnia catalysts are batch dependent. TABLE 8Impact of Hafnia on water gas shift catalytic activity at 240° C. OxideTOR²/ Catalyst Composition Pt wt % TOR¹/s⁻¹ s⁻¹ UR68Ce_(0.65)Zr_(0.35)O₂ 1.68 0.14 0.15 UR174 Ce_(0.65)Hf_(0.35)O₂ 2.69 0.130.19 UR149 Ce_(0.65)Hf_(0.35)O₂ 2.08 0.19 0.22¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂²1.5% CO, 45% H₂O, 25% H₂, 5% CO₂

EXAMPLE 43

The following example demonstrates the impact of platinum loading amounton water gas shift catalytic activity. Various catalysts were preparedaccording to the invention (Samples UR201A, UR201B, and Sample FF4 fromExample 27) and are given in Table 9. The data identifies the nominal Ptamount to be between 1-2.2%. TABLE 9 Impact of Pt loading on water gasshift catalytic activity at 240° C. Ce Zr Pt Catalyst atomic % atomic %wt % TOR¹/s⁻¹ TOR²/s⁻¹ UR201A 58 42 0.65 0.11 0.17 UR201B 58 42 1.120.17 0.22 FF4 58 42 2.16 0.16 0.25¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂

EXAMPLE 44

The following example demonstrates the effect of various noble metals onwater gas shift catalytic activity. Sample UR201A was prepared andloaded with platinum (Pt) as according to the invention. Sample UR201Hwas prepared and loaded with palladium (Pd) as according to theinvention. Sample UR205C was prepared and loaded with ruthenium (Ru) asaccording to the invention. Sample UR205D was prepared and loaded withrhodium (Rh) as according to the invention. Catalyst UR205E was preparedand loaded with iridium (Ir) as according to the invention. Clearly, thewater gas shift catalytic activity is maximized under the first set ofconditions when using Pt as shown in Table 10. TABLE 10 Impact of noblemetals on catalyst performance at 240° C. Noble metal Catalyst OxideComposition (%) TOR¹/s⁻¹ UR201A Ce_(0.58)Zr_(0.42)O₂ Pt (0.65) 0.11UR201H Ce_(0.58)Zr_(0.42)O₂ Pd (0.67) 0.01 UR205C Ce_(0.58)Zr_(0.42)O₂Ru (0.72) 0.01 UR205D Ce_(0.58)Zr_(0.42)O₂ Rh (0.88) 0.02 UR205ECe_(0.58)Zr_(0.42)O₂ Ir (0.59) 0.005¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂

EXAMPLE 45

The following example demonstrates the kinetic parameters for twoplatinum loaded ceria-zirconia catalyst samples. Table 11 compares thesteady-state kinetics for Sample UR129 (Pt/Ce_(0.25)Zr_(0.75)O₂) and FF4(Pt/Ce_(0.58)Zr_(0.42)O₂) that were prepared according to the invention.The experimental set up is identical to that described in Example 40.The parameters were fit to a simple model given by:Rate=A*exp(−Ea/RT)*[CO]^(a)*[H₂O]^(b)*[CO₂]^(c)*[H₂]^(d)*(1−β)  (1)

-   -   where β=([CO₂]*[H₂])/(K*[CO]*[H₂O]) is the approach to        equilibrium    -   A=Pre-exponential    -   Ea=Activation energy    -   T=Temperature    -   a=CO reaction order    -   b=H₂O reaction order    -   C=CO₂ reaction order

d=H₂ reaction order TABLE 11 Kinetic Parameters for Catalysts between240° C.-210° C. Parameter UR129 FF4 CO 0.07 0.08 H₂O 0.60 0.65 CO₂ −0.17−0.15 H₂ −0.68 −0.54 Ea/kcal mol⁻¹ 17 18 A 4 * 10⁷ 7 * 10⁷ R² 0.97 0.97

The results indicate that the H₂ order for Sample FF4 is greater thanthe order for Sample UR129. This explains the larger water gas shiftcatalytic activity for Sample FF4 compared to Sample UR129.

EXAMPLE 46

The following is an example demonstrating the effect of rhenium (Re)loading on the water gas shift catalytic activity of a ceria-zirconiananocrystalline support material already loaded with platinum, asdescribed in this invention. A platinum-loaded ceria-zirconia catalyst(Sample UR22A), prepared in accordance with the invention, was furtherloaded with rhenium as described in Example 38. Table 12 gives theturnover rates for this sample and compares it to a platinum-only loadedceria-zirconia catalyst (Sample UR68). Clearly the addition of rheniumprovides an enhancement to the water gas shift catalytic activity. TABLE12 Turnover Rate Comparisons Under 1.5% CO, 45% H2O, 5% Co2, 25% H2 anda Total Flow Rate of 2.6 L/min. Catalyst Moles CO/Moles Pt/SecRe/Pt/Ce_(0.58)Zr_(0.42)O₂ 0.325 Pt/Ce_(0.65)Zr_(0.35)O₂ 0.150

EXAMPLE 47

The following example demonstrates the impact of rhenium source on thewater gas shift catalytic activity. Various catalysts were prepared inaccordance with the invention (Samples UR235B, UR235C, UR235D) withconstant Ce:Zr ratios, loaded with platinum as according to theinvention, and then subsequently loaded with equal amounts of Re fromdifferent rhenium sources (nominally 3 weight percent) as described inExample 38. The rhenium source was rhenium carbonyl [Re₂(CO)₁₀] forSample UR235B, perrhenic acid [HReO₄] for Sample UR235C and ammoniumperrhenate [NH₄ReO₄] for Sample UR235D. The catalysts were tested in theexperimental set up described in Example 38. The results are given inTable 13 and compared to Catalyst FF4 that was not loaded with Re. Theimpact of varying Re sources is seen under two different feedcomposition at 240° C. The results demonstrate a clear promoter effectof the Re and also suggest no difference in activity when differentsources of Re are used. TABLE 13 Impact of Re and Re sources on catalystperformance at 240° C. Ce, Zr, Catalyst atomic % atomic % Pt wt %TOR¹/s⁻¹ TOR²/s⁻¹ FF4 58 42 2.16 0.16 0.25 UR235B 58 42 2.03 0.20 0.29UR235C 58 42 2.03 0.24 0.27 UR235D 58 42 2.03 0.22 0.28¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂²1.5% CO, 45% H₂O, 25% H₂, 5% CO₂

EXAMPLE 48

The following example demonstrates the impact of proper passivation onwater gas shift catalytic performance. Samples UR222A and UR222AB(Ce_(0.58)Zr_(0.42)O₂), both Pt loaded according to the invention, weresubsequently loaded with rhenium. Sample UR222A was prepared using theproper passivation protocol described in Example 38 while CatalystUR222AB was prepared according to Example 3 but without the passivationstep. The experimental set up was identical to that described in Example39. The water gas shift catalytic activity results are given in Table 14and clearly show that under varying feed compositions, Sample UR222Aperformes better due to proper passivation. TABLE 14 Impact of Repassivation on catalyst performance at 240° C. Ce, Zr, Pt TOR¹/ TOR²/TOR³/ TOR⁴/ Catalyst at % at % wt % s⁻¹ s⁻¹ s⁻¹ s⁻¹ UR222A 58 42 1.830.20 0.32 0.22 0.15 UR222AB 58 42 1.83 0.14 0.24 0.16 0.12¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂²1.5% CO, 45% H₂O, 25% H₂, 5% CO₂³2.1% CO, 22% H₂O, 11.2% H₂, 34.3% CO₂⁴1.5% CO, 33% H₂O, 30.3% H₂, 13.9% CO₂

EXAMPLE 49

The following example demonstrates the steady-state kinetics for aplatinum-only loaded ceria-zirconia catalyst and platinum and rheniumloaded ceria-zirconia catalyst. Samples FF4 and UR235B were prepared andloaded with platinum as described in the invention. Sample UR235B wassubsequently loaded with rhenium as described in Example 38. Theexperimental set up was identical to that described in Example 40. Theparameters were fit to a simple model given by:Rate=A*exp(−Ea/RT)*[CO]^(a)*[H₂O]^(b)*[CO₂]^(c)*[H₂]^(d)*(1−β)  (1)

-   -   where β=([CO₂]*[H₂])/(K*[CO]*[H₂O]) is the approach to        equilibrium    -   A=Pre-exponential    -   Ea=Activation energy    -   T=Temperature    -   a=CO reaction order    -   b=H₂O reaction order    -   c=CO₂ reaction order    -   d=H₂ reaction order

The results for both catalysts are given in Table 15 and indicate thatthe positive effect of Re is due to a larger dependency on water withlower inhibitory effects from CO₂ and H₂. TABLE 15 Kinetic Parametersfor Catalysts between 240° C.-210° C. Parameter FF4 UR235B CO 0.07 −0.05H₂O 0.67 0.85 CO₂ −0.16 −0.05 H₂ −0.57 −0.32 Ea/kcal mol⁻¹ 17 17 A 2.5 *10⁷ 4.5 * 10⁶ R² 0.97 0.93

EXAMPLE 50

The following example demonstrates the impact of surface area on watergas shift catalytic activity. Sample UR59B (CeO₂) was synthesized andloaded with platinum according to the invention and represents a highsurface area material. Sample ULSAC3, which represents a low surfacearea material, was prepared by a method not in accordance with theinvention, but Pt loaded according to the invention. The experimentalsetup for catalytic testing was identical to that described in Example40. Table 16 demonstrates that with a larger surface area (UR59B), theactivity under the first set of conditions is nearly double that of thelow surface area sample (ULSAC3). TABLE 16 Impact of surface area onwater gas shift catalytic activity at 240° C. Oxide Surface CatalystComposition Pt wt % area, m² g⁻¹ TOR¹/s⁻¹ UR59B CeO₂ 2.21 162 0.030ULSAC3 CeO₂ 1.00 23 0.017¹4.9% CO, 33% H₂O, 30.3% H₂, 10.5% CO₂

Although the invention has been described and illustrated with respectto the exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the spiritand scope of the invention.

1. The method of optimizing water gas shift activity for a water gasshift reaction on a reformate in the presence of a shift catalyst,comprising the steps of: a) determining certain compositionalcharacteristics of one or more reformates for a range of reformatecompositions comprising a reformate range of interest; b) determiningthe respective activity rates for a range of shift catalyst compositionsrelative to the reformate range of interest; and c) selecting for thewater gas shift reaction, from the range of shift catalyst compositions,a shift catalyst composition having a favorable activity rate for thereformate range of interest or alternatively, from the reformate rangeof interest, a reformate composition providing a favorable activity rateto a predetermined shift catalyst composition.
 2. The method of claim 1wherein the range of shift catalyst compositions is selected from thegroup consisting of: a range of ratios of Ce to one or both of Zr and Hffor a ceria-based mixed-metal oxide, a range of ratios of Pt to Re metalloadings, and a range of ratios of combined Pt and Re metal loadings toa ceria-based mixed-metal oxide.
 3. The method of claim 2 wherein saidratio of Ce to one or both of Zr and Hf is based on atomic % and saidratio of Pt to Re is based on wt %.
 4. The method of claim 1 wherein asaid favorable activity rate comprises substantially the highestactivity rate determined for the reformate range of interest.
 5. Themethod of claim 1 wherein the range of reformate compositions includes arange of H₂O to CO ratios.
 6. The method of claim 5, including thefurther steps of determining if one or more of said H₂O to CO ratios isgreater than six (6) and if so, examining a range of noble metal-loadedceria-based mixed-metal oxide catalysts for respective activity ratesand selecting the said catalyst having the highest activity rate.
 7. Themethod of optimizing water gas shift activity for a water gas shiftreaction where a reformate is to be flowed across a catalyst bed,comprising the steps of: a) selecting a plurality of regions, insequence, in the direction of reformate flow across the catalyst bed; b)separately for each of said catalyst bed regions: i) determining certaincompositional characteristics for the reformate to be flowed across therespective catalyst bed regions to define a reformate range of interest;and ii) determining the respective activity rates for a range of shiftcatalyst compositions relative to the reformate range of interest forthe respective catalyst bed regions; and c) selecting for each saidcatalyst bed region from the respective range of shift catalystcompositions, a shift catalyst having a favorable activity rate for therespective reformate range of interest, for optimizing the water gasshift activity across the catalyst bed.